0👍
I think any UI library is mostly concerned with the framework specific way of handling things like view updates, rendering, and styling. The frameworks have different concepts regarding those things and therefore individual libs are often necessary to utilize a frameworks ability. When something is being rendered differences in the frameworks have to be taken into account. Therefore you often see library versions for each FE-framework
On the other side, Javascript will always work and therefore a library which bundles functionality (e.g. moment, lodash) can be used in every framework in usually the same way.
If you want to share UI components I strongly recommend using web components which solve framework specific problems like styling as a web standard. You can use them with every framework (e.g Ionic).
In conclusion: JS libraries which aren’t concerned with rendering can usually be reused. When it comes to UI libraries, like bootstrap as you mentioned, components often have to be written for each framework. Web components can be a solution because they work with every framework as well as vanilla JS.