Its good practise to use some theme as starting point and then modify it yourself or it is considered too much work in maintaining that and better to invest time in searching for more suitable theme which will not need modifications?
Try not to edit the theme files directly unless you plan to make it your own theme decoupled from upstream.
If you need to edit a file make a copy of it and place it in the root of your site, root files always have precedence over theme files. It’s also possible to have a list of themes and Hugo will pick from them in order.
It’s also possible to have a list of themes and Hugo will pick from them in order.
If I want to use front page from theme A and posts from theme B.
I will list theme A first in configuration and then delete what I do not want from theme A in themes/A manually?
If you use ready made themes I would avoid editing them in any way. It makes it hard to update. Do all your edits in your own theme or in root files.
If you simply want a file or two from themes I suggest copying them to your project root or to your own theme.