If my phone natively supports Widevine L1 in order to play 1080p to 4K videos on Netflix or Amazon Prime, will Netflix play the high res rather than 480p with magisk that passes safetynet.
Unlikely. Generally, on most phones, your DRM keys will be wiped when you unlock the bootloader. So, even if you clear SafetyNet, your Widevine level is most likely already dropped down to L3. That means no more HD playback. Locking the bootloaderback will work on some phones, won't on others. Ideally, it is a permanent loss of Widevine L1 when you unlock with no way to restore it.
I am not sure if there are some hacks which pass fake Widevine status to the apps in question and restore their HD playback.