Question about Grids and Layouts in UI Design

grids and layouts are something that i've struggled with since starting my design career two years back. I've read books like grid systems and watched several videos and courses but as a UI designer, it feels like a lot of these resources aren't particularly catered towards my profession. Layout Essentials is next on my reading list, but judging from a quick flip through, looks to be still catered for graphic design.

I understand that grids for web design are fairly simple, and whenever in doubt, I can always fall back on the bootstrap columns, I'm not particularly sure about other cases studies such as desktop, TV or even car UI design. I get that we just need to define a column size, some dimensions for our margins and gutters and that's it, but my issue is coming up with these specific numbers in the first place. Any number I come up with just feels very arbitrary even when catered to the use case.

Wondering if someone could share their process on defining a grid when designing a desktop app, mobile app or website, or if there are any good books that can be recommended for grid systems in UI design specifically.