In many cases, it’s the same thing. PKGBUILDs from AUR simply automate the process.
Read @linux-aarhus’ guide for details.
Prefer installing packages from Manjaro’s repositories, as much as possible. Moreover, all editions ship pamac for managing packages graphically, and third-party sources are disabled by default.
AUR, Flatpak and Snap should rather be restricted for packages unavailable in the repositories, or for some specific cases.