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