If macOS is allowed to run without interruption for an extended period of time, it is possible that the processes controlling the webcam may get hinky. I am unclear on why exactly this happens.
Two commands can resolve this, depending on which process failed.
sudo killall VDCAssistant
sudo killall AppleCameraAssistant
TL;DR: Use pamac
The AUR is outside of pacman’s scope, as it is not really a “repository” in the same way that the arch or manjaro repos are. It instead contains individual git repos and PKGBUILDS, which describe to pacman how and where to grab (and/or build) binary packages. the archwiki details several AUR helper programs, which either wrap around pacman and provide searching of the AUR, or similarly wrangle grabbing the git details from AUR and shoving them into pacman.
pamac is one such tool built by the Manjaro team. It has a GUI program and a CLI program. These can be configured to use the AUR. It also supports flatpack, snap, etc, and has multithreading downloads built in.