(in short duplicate layer, mask the subject at top layer and fill the bottom layer with full background)
1. Duplicate Layer (Ctrl+Alt+D or from Menu Bar>Layer>Duplicate Layer
   
2. Select Subject (via magic wand tool [w] or MenuBar>Select>Subject)
   
3. Either copy the layer (ctrl+J) or create a mask for it from the Layers tab -- Finetune the masking if needed since Psp selection is prone to mistake.
   
4. Select the original (bottom) layer, hold Ctrl and clik the mask layer (top layer) to get the Subject selection again
   
5. Expand Selection by around 30px (Select>Modify>Expand)
   
6. Use Content Aware Fill (shift+backspace or Edit>Fill)
   
7. Make the layer smart object (Layer>Smart Object>Convert to Smart Object)
   
   
   
   (create blur filter, set the blur and bokeh from the right panel)
8. Apply FIeld Blur (Filter>Blur Gallery>Field Blur) for a flat blur, or Use Tilt Shift for gradient Blur
   
9. Tinker with the Blur slider. if you want to add Bokeh, tinker with the Effect tab below it
   
   
10. commit the filter and the image should be done
    

limitation note:
If the game doesn't use Unreal Engine or its camera isn't listed on this site, then it won't work. The camera tools are required to make the DoF function.
https://opm.fransbouma.com/Gamespecificfunctionality.htm

note: i've learnt this technique, but i prefer the photoshop one since it technically support any game. But i'm leaving my research notes here for future me/ people interested.

install reshade:
https://reshade.me/
install otis camera connector
https://github.com/FransBouma/IgcsConnector/releases
Unreal Engine Unlocker
https://framedsc.com/GeneralGuides/universal_ue4_consoleunlocker.htm
newer unlocker, UE5 unlocker, certain games need to subscribe to his Patreon
https://www.patreon.com/Otis_Inf


tutorial on how to use the tool:
https://opm.fransbouma.com/igcsdof.htm
video tutorial (not mine)
https://www.youtube.com/watch?v=NLZeFKyXeF4&t=322s

technically Photoshop is faster than Reshade way, and it supports all games.
For Otis tool, technically you can subscribe for a month, take all the tools you need and unsub for the months after (If you like the tool, pls support the author tho).
BUT
since we use selection tool for the subject in photoshop, fine tuning the selection (depending on the complexity of the subject) can take longer.

If you want the 'correct' result, i'd suggest using the Reshade way since technically it separates the subject and background from the game itself. Though on Photoshop you can create the 'perfect' result catering to what you want, ie. you want to blur certain parts, custom gradient blur, etc

All my screenarcher related guide can be accessed here
https://steamhost.cn/steamcommunity_com/groups/SAtips/discussions/1/
If you join the group, you can access more stuff too here
https://steamhost.cn/steamcommunity_com/groups/SAtips/discussions/0/

Feel free to drop a comment or question if you have any, i appreciate any support, feedback, and i would encourage you to share your tips and findings too !!

as commented below by @COMMANDER STEFFA™

there is some other method, like Nvidia Ansel, (maybe AMD also has similar feature)
In supported games just invoke it with Alt+F2 and select "depth of field" to have fantastic control over the focus!

Or, another method yet, as Miranda told us in his Meet the Author consists of using Adobe Lightroom where Artificial Intelligence will also help us in this process!
btw, shoutout to "Meet the Author" which is an amazing medium commandeered by Steffa. here's the link :
https://steamhost.cn/steamcommunity_com/groups/CommSteffaFavScreenshots/announcements/detail/4526772422647237627

love this kind of feedback! ❤︎