So, I'm a backend developer, wanting to get into frontend development so that I can be a full stack dev. I've watched and done some tutorials on html/css about flexbox/grid. Should I use Bootstrap due to its compatibility, or stick with flexbox/grid? What's the pro/cons of using bootstrap vs flexbox/grid?

I'd suggest trying to build up a few front-end components from scratch on your own. Try your hand at menus, accordions, and even modals. See how difficult it is. CSSTricks is an amazing resource for learning this kind of stuff. Really get to know all the issues.

Once you understand how many problems Bootstrap and other similar frameworks solve, you can use it on a case by case basis. If you need to get a large app completed in a limited amount of time, it can shave months off your development time. However, there are costs when it comes to how easy it is to customize and how large the library itself is. Also, when you run into css bugs or want to change a component to work a little differently, the foundational knowledge you established early on will come in super handy.

As a full stack dev, I personally prefer starting with a minimal toolset, like a css reset and some small css utility libraries. That keeps things simple and I can make sure I don't run into any big surprises. It also matters a lot because I like to keep my designs "pixel perfect", so the odd padding or random margin can really hamper my flow. However, if I ever needed to build out an internal-use only admin interface -- or if someone hired me to build an MVP in under 3 months -- I wouldn't hesitate to turn to bootstrap (or a similar framework).

Bootstrap is not just about making layout, it has so many ready to use components. Which make your buttons, forms look better and give you components like modals, popover etc

If you are using some framework specific CSS library (meterial-ui for react, primeng etc) you may not need bootstrap you can just use flex/grid + framework CSS library

Bootstrap uses flexbox under the hood. Ideally you should understand both (in the same way you should understand JS before using jQuery).

Bootstrap just gives you a lot of battle-tested components out of the box (like modals for example), it's not a replacement for CSS.

Congrats team! However, since I've gotten lost in a flood of nostalgia by listening to "Nothing's gonna stop us now", I will have to wait until later to seriously read the release notes.