Greetings, NerfPablo!
Welcome to Apple Support Communities! If we understand correctly, you are running into issues with the iPhone volume automatically lowering when you take off your AirPods or when you switch between music and videos. We are glad that you reached out to the Communities for help. We will do all we can to help you locate the answers you need.
First, we'd like to ask for a few details about the issue itself. What type of iPhone do you use? Which version of the software is on the iPhone? Does the problem occur if you use another Bluetooth headset or set of AirPods with your iPhone? If you're unsure (and it is possible), please test. Does the issue appear to be app-specific? If so, which app(s)?
Also, you mentioned that the issue occurs when you're switching between sound and music. Does this happen only when you're using AirPods? Or, does it also happen when the AirPods are disconnected and you are listening to video/music from the built-in iPhone speaker? If it only happens with the AirPods, you may want to try to reset your AirPods. If you're unfamiliar with those steps, you can look them over here: How to reset your AirPods and AirPods Pro.
We hope this information helps you out.
Take care!