I picked up a decent understanding of who the Remnant were and what their motivations are through the game. If anything I was let down by how trope-y their backstory is but there was quite a lot there.
Basically the Remnant were trying to seed their galaxy with intelligent life. Part of that was in terraforming a lot of marginal worlds and they set about building a galaxy-spanning terraforming network to do that. Then they were suddenly attacked by an extra-galactic enemy that tried to turn their terraforming tools into weapons with partial success. The attack of the extra-galactic enemy coincided with the appearance of the scourge, though this is muddied because the Angarans also report that the scourge first appeared during their time many millennia later. Perhaps there was a subsidence of the scourge between when the Remnants were fighting the galactic menace and later when the Angarans had their society destroyed by a resurgent scourge. Or perhaps it's sloppy writing or maybe it will be explained when I finish the game. It is also unclear if the extra-gallactic menace caused the scourge or if it was purely coincidental, though this is an intentional obfuscation by the writers as they were probably saving that story arc for DLC or a sequel.
Anyways, the Remnant were basically driven to extinction by this extra-galactic menace and their terraforming systems were thrown into disarray and in some instances turned hostile. The Angarans built their society in the ruins of the Remnant and could manipulate that technology to some extent. Then the scourge showed up (or rather re-appeared) and massively disrupted the Angaran civilization. Concurrently, the Kett (who are also possibly extra-galactic) showed up and took advantage of the weakened state of the Angarans to enslave and mutate them into Kett soldiers. During this process, the Kett realized the Remnant terraforming network could be weaponized and began searching for the key to operating that network.
Onto this scene, the Milky Way crew crash-lands into the galaxy and further stirs the pot. I also believe the game hints that the Angarrans themselves might be the Remnant but I'm unsure.
I think the problem is not that the game doesn't answer questions, it's that it floods you with a ton of extraneous world-building information and does so in such a way that you can't easily pick out what's part of the overall story arc versus what is just background filler. This obfuscates a lot of the information. Also, to get this level of understanding, you have to sink a ton of time into all of the side quests as they droll out this important information as much in the side quests as in the main story. For past ME games, this approach worked because there simply wasn't as many side quests. I'd say MEA has as many side quests as 2 or even all 3 of the ME mainline games combined - and definitely as much lore as those other games combined.
I got all of that information from the game itself, I have not done any web searches for supplementary material or fandom guides.
One other brilliant/terrible way this game does story telling is through informal interactions. All of the characters, from your core team to the random NPC on the street corner, are extremely chatty. A lot of core information about the main story arc is only revealed through random chatter between characters in the background. This is brilliant because it does make the game world feel more alive and real but it sucks because when you're already having to have 10 million conversations and read multiple long encyclopedia articles to learn about the world, you're going to mentally filter out a lot of this chatter. It's overwhelming.
On Fortnite - I'm unaware of any zombies in that game but I don't know much about it. I do know it is like how I described though.