i had the same exact opinion about myself until i tried starting a project with just bootstrap's reboot css file and realized that was enough to get a nice style and build on top of that
I need to sit down and learn flex css property so I can replace bootstrap row and col with flex.
Neat! I have been using Bootstrap for several years at work and for personal projects. It allows non-ux folks like me to build useable apps quickly. So thank you!
If you are looking for a quick way to get up to speed with flexbox or css-grid you should check out
https://flexboxfroggy.com/
and
https://cssgridgarden.com/
They are fun little tutorial games that introduce the basics
Why not both? Grid is superior for defining the overall structure of a page, but flex shines in micro-layouting. (Although the line between them is blurry sometimes).