Really nobody know?

I believe you can only do that with your own cities unfortunately. There appears to be limitations with CIV 6 that didn't exist with 5.

how do i select my own city?
I mean when i click a city, the city screen opens?

you need to set a hot key for the cheat option you want to use in the city. for example, I set the Delete key to "add 1 pop to selected city" and then when in the city menu, I can hit delete and the pop increases.

as mentioned by Mr. Cloudies, you probably cannot affect other cities, I would base this on the way Civ 5 works, there is a permissions configuration in the game database that assigns ownership and access rights to every object on the map (like cities) and this limits what you can do to them. The cheat menu for Civ 5 has an option to take ownership of the city, which then allowed you to make modifications to it, and if you wish, you could give the city to any other civ as well.

finndo-eewta の投稿を引用：

you need to set a hot key for the cheat option you want to use in the city. for example, I set the Delete key to "add 1 pop to selected city" and then when in the city menu, I can hit delete and the pop increases.

as mentioned by Mr. Cloudies, you probably cannot affect other cities, I would base this on the way Civ 5 works, there is a permissions configuration in the game database that assigns ownership and access rights to every object on the map (like cities) and this limits what you can do to them. The cheat menu for Civ 5 has an option to take ownership of the city, which then allowed you to make modifications to it, and if you wish, you could give the city to any other civ as well.

How did you make the hot key?

Came here for this. In Keybindings Duplicate and Destroy cities is not an option. Anyone figure this out?

It's simple. First, you have to own the city and it can't be your capital.
next, open the cheat Panel at the top left.
Then click on the city you want to destroy, so the city panel shows up with the name of the city you've selected.
then click the "Destroy City" button ( icon of city with red circle like 'no smoking') in the Cheat Panel.
I've done it and did not set it to a hotkey.