Especially for things used directly, you need to understand both, own and third party code, roughly to the same level. With own code, you only care for your own use case; with third-party — you have to kind of get everyone else's.
Depending on what you do and the dependency's scope, either way can make sense.
Third party dependencies are very easy: you just have to intimately know how it is implemented in addition to knowing your own code and stack, and then you are golden!
Nothing to learn, just focus on making your app, it’s all taken care of by This One Simple Package ;)
These things are so far from free as our tooling presents with “just nuget it or whatever”.
Classic case of "focus on building your app, not infrastructure". Here's another multi-million dollar idea: put this cache directly inside your own video processing server and upload there.