Forgetting to change your resolution or setting up your mic will result in an unusable lesson. So please, please, please, set your resolution to 1280x720 (HiDPI), set your mic input to "Onyx Blackjack", and confirm that your mic loud enough (it helps to put your mouth as close to the mic as possible).
The app for changing the resolution that is being used is SwitchResX from http://www.madrau.com/
Thanks Anzumana, spent 30 mins trying to figure out how to set the resolution correctly.
If you want, you can use EasyRes (https://apps.apple.com/us/app/easyres/id688211836) instead of SwitchResX. It's free.