@gn Safari for larger websites or watching videos (same for QuickTime).
These days barely touch Adobe software but back in the day I used to fullscreen After Effects and Premiere while Photoshop or Illustrator would depending what I was working (I assume it might be due to the timeline and AE layers, never enough space).
Figma/Sketch fullscreen sometimes (depending on what I’m working).
Not sure why, lately feels nice to have a sneak peak of the desktop wallpaper so I fullscreen even less 😅
@gn Slack is the only one that's full screen because I don't want it participating in normal window management.
Others I have sized to use a full monitor, but I use Spaces extensively and usually group multiple windows (IDE) based on domain. So then I am only switching between a handful of spaces and then cmd+tilde between windows on those spaces. This is all on 1 monitor (my "main).
Ancillary monitors have web browsers, DB tool, postman and other reference, sometimes Netflix.