I don’t know about videos but having a look at the OSI model is a good way to start. It covers the abstract framework for packetizing data including things like the distinction between hardware and software, envelope, encryption, application layer stuff, the whole shebang. The cool thing is by going hardware, network, application you can see where responsibility are and it helps you understand where things can go wrong.

If you are interested there are plenty of CCNA style courses available on the internet, licit and otherwise, and they go into more depth, and the same applies to RHCE/RHCSA material. The training for certifications like that covers what you want to know but also puts it in context, and again licit and otherwise sources are available.

Root your phone and you can manage which APN is used by tethering. If you can’t do this consider trying a connecting to a VPN before enabling tethering, the connection will on some devices remain active on the normal APN because changing would disconnect the VPN and keeping connected is higher priority than updating the APN. Also USB tethering and WIFI tethering may behave differently.

In the end this is a good argument for better regulation. When you buy a car they don’t get to extract more money from you because you drive out of state or use it for business. The fact that telecommunications companies have so much power and access to basically monitor what you are doing and bill accordingly is insane. You should pay for a service with a simple and clear contract and all this crap should be made illegal.

It looks like it is downsampling the video or streaming after converting to another codec. Some codecs are fine for decoding on the server but the app may not support them so the server converts them. Some files are of higher quality than what the server is configured to deliver so it downsamples to stream it.

Check the configuration and look for anything to do with codecs, hardware decoding, streaming quality, and so on. It may also be on the app, so if you can access a different interface then test that to narrow down the issue.

Something I have found is missing from both of these suggestions as well as every podcast app on device is transcoding to speed up so it is not sped up on the fly. For a lot of phones and other devices the task of playing back at 2x speed is enough to demand a higher power state than what is required to play a sped up file. For efficiency doing a single pass of speeding up the audio then playing back at that speed would use less power during the playback phase, allowing you to download and speed up all of your podcasts at home while on charge then listen for long periods without completely killing the battery. I have checked with a few if the open source devs and this is not a feature they see utility for so nobody intends to make it.