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The name of the game is ".atorb.".
It's unusual for the name of a game to start with a period (a full stop), but possibly it was called that so that it would appear at the start of an A-Z listing of games on Steam (and elsewhere).
.atorb. is an ok game for a bit of time-wasting (or for testing your aiming/coordination skills).

The game has two modes, standard and infinity, with three types of difficulty (easy, normal, and hard).
Standard mode has 127 levels on the easy and normal settings, and 126 levels on the hard setting.
Infinity mode never ends.

Each level has one outside ring, with some inside rings.
A series of balls are shot from the outside ring to the inside rings.

The default shooting position is on the outside ring on the right side, at the 3 o'clock position.
The shooting position on the outside ring can be changed by moving your mouse left (to move clockwise) or right (to move counter-clockwise).

The inner circles change direction every so often, which is not a big problem on easy or normal difficulty, but can be a real pain on hard difficulty.


Notes
Whilst this game is no longer available to purchase on Steam, it still has a page on the Steam site, with various bits of information:
https://steamhost.cn/steamcommunity_com/app/467530

The developer of the game is Vasily Bulygin.
The game is no longer available on Steam, as there were some problems between Steam and the developer (for allegedly violating Steam's terms of service), resulting in the developer's games being pulled from sale on the Steam store (although people who already bought those games on Steam can still play them).
As the developer is no longer able to sell content on Steam, this means that if you want to play this game, you will have to get it elsewhere, or hope that it comes back to Steam.
https://steamhost.cn/steamcommunity_com/games/467530/announcements/detail/529570286905603455

Each coloured ball must land on the inside ring which matches its colour.
If a ball hits another ball it will disappear, and will re-queue in the line of balls waiting to be shot.

This difficulty level is not just easy, it is extremely easy.
You can pretty much just close your eyes and just keep on clicking your mouse to get all of the balls onto the inner circles.

About the only time that shooting with your eyes closed won't work is if the balls have filled up so much room on the circle that, even with the circle moving about, you can't fit any more on when shooting from the default position.
Naturally, in that case, just move your shooting position, and fire away again. Too easy.

Each coloured ball must land inside an empty circle on the inside ring which matches its colour.
If a ball misses, or hits another ball, it will disappear, and will re-queue in the line of balls waiting to be shot.

Just line up your ball with the empty circles, and shoot.
You will need to judge the movement of your target, as compared with the position and speed of the shooting ball.

Each coloured ball must land inside an empty circle on the inside ring which matches its colour.
If a ball misses, or hits another ball, that level will have to be started again.

As the inner circles change direction every so often, then - when you have almost completed a level - it is worthwhile waiting for the direction to change before taking a shot, as if you miss (due to a change in direction) you will have to restart the level.

Having to restart a level when you have almost completed it can be a real pain, so be careful with your shooting.

It's easy and it's first
Complete first level on easy mode
-- Finish one level on the "easy" difficulty setting.

42
Complete level 42
-- Finish level 42.
This is easiest to complete on the "easy" difficulty setting.

Not so hard
Complete first level on hard mode
-- Finish one level on the "hard" difficulty setting.

Why so normal?
Complete first level on normal mode
-- Finish one level on the "normal" difficulty setting.

Lucky throw
Throw circles 777 times
-- Shoot the ball/orb/circle 777 times.
Just keep playing and you will get this.
Here's some historical information regarding the number chosen for this achievement:
Why 777? Well, the number 7 is regarded as lucky in modern popular culture, as well as in several ancient cultures; in some Christian traditions 777 is believed to be a "holy" number, which opposes the "unholy" number of 666, the latter being the number of the Beast, derived from Revelation 13:18 in the Bible, i.e. "Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six" (King James Bible).

So easy
Complete all levels on easy mode
-- Finish all of the levels on the "easy" difficulty setting.

Nucleus collider
Miss 100 times
-- Miss the ring (on easy) or miss the empty circles (on normal or hard) 100 times.
This is easiest to complete on the "normal" difficulty setting (on "easy" almost every shot will hit; and on "hard" the level restarts every time you miss, which would make missing 100 times take quite a while).

Normalizer
Complete all levels on normal mode
-- Finish all of the levels on the "normal" difficulty setting.

Goose is back
Find goose
-- To get this achievement you have to complete all of the levels on easy mode and normal mode; and then, in hard mode, get to level 7 and push the "G" key 15 times.
The information on how to get this achievement came from a post by the developer.
See: https://steamhost.cn/steamcommunity_com/app/467530/discussions/0/364041432729437033/
Note: If you want to remove the goose from your screen, just go to the "Options" screen, and turn "Companion" off.

Go hard
Complete all levels on hard mode
-- This is the hardest of all of the achievements.
See the notes in the "Basic gameplay" section on how to handle the game on hard difficulty.
Most of all, it will take aiming skill and patience.

Note:
Just in case you were wondering, the three achievements "Lucky throw", "Nucleus collider", and "Goose is back" all have a blank circle for their achievement icon.
Maybe the developer meant to do an individual icon for each one, but never got around to it?

Be warned that Atorb does not have a specific "save game" feature.
Whilst it is supposedly set up to automatically save your progress, sheol happens.

I had the experience of getting up to Level 119 on the hard difficulty and then the next time I went to play the game I was back to Level 1 again (I was cheesed off, to say the least).

I had a look at the game's folder and it appears that the following file is the one that saves the game data:
C:\Program Files (x86)\Steam\SteamApps\common\atorb\atorb_Data\output_log.txt
So, to save my game progress, I copied that file every so often, to back up my progress.
As it turns out, the problem didn't happen again, so I didn't have to load that file.
If my assumption is correct, and that is the game's data file, then I hope that solution may save others from the same problem that I had.

However, I finished the game in the end. Woot!