During my career, I've seen a lot of approaches when styling a website: using a popular framework (Bootstrap, Tailwind, Skeleton), combining a popular framework with a custom toolkit, using your own custom-built framework, etc.

What are your thoughts behind this? When to use one, and when to use the other?

Personally, I've not been using frameworks lately at all, as I've become more experienced and proficient with CSS. But I try to keep it as simple and maintainable as possible, so I do not build a complicated build chain (with Gulp) and I use npm handle the updates.