It’s because all the good codecs (aptX, LDAC, even SBC) all operate over the A2DP Bluetooth profile, meaning your computer’s Bluetooth adapter is running in a particlar mode called A2DP. Unfortunately, A2DP is unidirectional, (ie, not Duplex) it doesn’t support sending audio to headphones and receiving audio from the mic at the same time, due to bandwidth limitations. So when you open Zoom or Teams or something that needs to access your mic, your Bluetooth adapter switches to a different mode like HSP (Handset Profile) and HFP (Hands Free Profile). These profiles do support duplex connection to the mic and headphones, but don’t have fancy high definition codecs. They are designed to be very low bitrate. That’s why you encounter such a big audio quality difference when it switches.
Note, having said that, pipewire on Linux does support some fancy faststream duplex modes that operate on A2DP, outside the constraints of what I explained above. Eg, it gives me the option to enable AptX-LL with faststream, that passes the mic through using whatever little leftover bandwith is on the connection. This doesn’t work with heavier codecs like AptX-HD. I think Android does something similar, that is why you don’t notice the profile swapping behaviour as badly on Android.