Is it possible to set opening certain apps such as Safari, iWork, MS Office in the full Window mode by default like Mail App (macOS Catalina)? The only App I can set opening in the full window is Apple's Mail App - it can be set in the App setting. I suppose other application could be set via terminal commands, but I'm not that handy with terminal commands to do so.
Yes you can open any app in full window

Use keyboard shortcut 

Command + Control + F

Same to exit full window as well