Difference between Vortex, BG3 MM, and manual easiest answer is organization. Installing manually you have to organize the mods yourself, I do this personally for sims 4 if I get my building itch, sims 4 load order is alphabetically and I ran into a problem where it looked like Vortex did not do this correctly.
Vortex also when I was digging around looked like it kept three copies of one mod on your computer (this one I could be wrong and its just two but I'm pretty sure they install a backup) one is the installed and the second is backup, for the third is the one the app uses to install your mod.
I have no experience with BG3 MM but here are two vids
I actually learned about modding back when skyrim first was out, Gophers channel had vids that broke down the ins and outs of the whole process. He'll have premade playlists on his channel for modding other games but it might help understand additional info about modding totally. First is for vortex, second is Gophers main page for his youtube channel.
