Some questions about the plot (Spoilers)

1 is kind of a big spoiler, but you were supposed to know it anyway, so

1. Fou is actually Beast IV, one of the calamities that threatens human history (same as Goetia). He trades his intellect and individuality, everything that makes who he is, for Mash's second chance (for some reason they decided to not show it on the anime version).

2. The servants summoned were all servants that Ritsuka encountered during his journey to save humanity. They came by themselves with no need for summoning, kind of a miracle thing to happen.

3. I'll leave this one for someone else because I honestly don't remember. I think he was spamming command spells and Goetia was weak af.

Don't trust me 100% on those because it's been a while. I remember the general idea, but not the small details.

the first one was explained so I'll just focus on the other 2.

the fate grand order animes don't show the full story so all the servants that appear are one that fujimaru met during the events of the game for singularity 1 to 7. and basically fujimaru is like the perfect master candidate just for the way he bonds easily with his servants and as the story goes on and he meets and befriends more and more servants he weaves threads of fate with them which when he enters the temple of time which exists (ironically) outside of time the servants can summon themselves to help him based on those connections he made. cheesy? yes. plot hole or armour? no it's kinda the whole point of fate. its based around the characters and the connections they make between master and servant as so on.

as for how he's fighting goetia basically 3 major factors
1. Dr roman/ solomon majorly nerfs goetia with the noble phantasm that's why goetias freaking out so much.
2. fujimaru can do basic mage craft and I think heals his nervous system, which was being used to summon the 7 servants instead of magic circuits cause his are really weak, and then activates them but on himself which gives him a strength boost allowing him to pick up the shield and stuff
3. Edmond dantes- the servants you see appear behind fujimaru at the start of the fight. he's a key individual in the story and basically protects fujimaru from going insane from mind attacks and just the entire ordeal he's going through. in the fight he physically enhances fujimaru allowing him to float and what not and actually have somewhat equal footing
and also technically 4. there's still a bunch more servants who fight alongside fujimaru against goetia like scathatch and David

I might of got some things wrong but the main points are definitely correct
so yeh I don't blame you for not understanding, the film whilst still enjoyable to watch is a pretty mid adaptation of the game so not everything is explained properly and it was definitely mainly made as fanservice for fgo players.

all the animes that mash and fujimaru appear in respectively are

fate grand order: first order 1 movie
fate grand order: camelot 2 movies
fate grand order: babylonia 21 episodes
fate grand order: solomon 1 movie
fate grand order: moonlight lostroom 1 ova

moonlight lostroom will not make sense whatsoever though and is made as bridge from part 1 to part 2 of the game but for anime onlys it's terrible so you don't have to watch it.
but yeh they are the only animes that those 2 appear and currently there are no plans to animate any more, if you want to see the rest of their adventures and story you'd have to play the mobile game. the anime has probably shown like 20% of their story and the adaptations themselves aren't very good so I do recommend the game but I understand if you don't want to play it.