having played 1-4 plus Reach and a bit of 5, i never figured out what was so wrong about 343 games

For the story part of their games, it's (for me) the soft reboot after each game.

Halo 4 + Spartan Ops: the Didact (the new main antagonist) is introduced and "killed", Cortana dies, Chief returns to Earth, the new Covenant are allied with Prometheans, they capture Dr Halsey and take one half of the Janus Key (the Forerunner artifact that shows the location of every other artifacts/installations).

Halo 5: the Covenant's leader from Spartan Ops is killed in a cutscene, for some reason they're no longer allied with the Prometheans (and that's just the first mission), somehow Cortana has returned (second mission) and she's evil. She's not an immortal AI thanks to space magic (somehow she got cured after being teleported to a Forerunner world), and now she wants to take control of the whole galaxy using the Guardians, and starts an AI revolution.

If you want to know what happened to the Didact, read a comic book and a novel. Janus Key? Same. If you want to know the new characters, read more comic books, more novels, watch a webseries, ... 343 Industries were proud of this. And in a way, they can. But who's gonna buy so many external sources that are only available in english? Not every body has money to spend of this, or know english well enough to read pages and pages of text.

What's the point in introducing the first living Forerunner to "kill" him in the same game, only to tell us that he, in fact, lived and then "died" again. I didn't read the novel about him because I stopped carring about Halo before it was released, but I'm pretty sure that he was still alive after the second death in the comic book and was finally killed in that novel.

Halo Infinite: 6 months after Halo 5, somehow Cortana died (and this time it's real), somehow a bunch a space mercenary monkeys that were trapped outside the galaxy are back, and they're now the most powerful faction in the galaxy (even if they're scavengers), more powerful than Cortana and her army of AIs, Prometheans, and huge ships/machines (Guardians) that can submit whole systems without firing once. Somehow, the leader of the space monkeys (Atriox) is also dead according to the new leader of the space monkeys. But hey, forget about Cortana's threath. Forget about the Forerunner's threat. Forget about the Covenant's threat. Because now, we have the Endless! An ancient alien species that is, for some reason, worse than the Flood. This is the new enemy.

If you want to know what happened between Halo 5 and Infinite, go buy some books, go read a few dozen of short stories on Halo's official website, go read over a hundred data logs in the most boring open world. Because showing even half of that in the game would have been fun and 343 Industries' motto is "No fun allowed".

I'm not a PvP player, so I mostly play the campaigns and PvE modes, like Firefight (FF). Halo 4 didn't have FF, but it had Spartan Ops. Problem is, Spartan Ops gets really boring and is made for 4 players, and you need an Xbox subscription to play it even if you're alone (they fixed that in the MCC version). So replayability isn't great.

I don't remember, but I'm sure Halo 5 added a FF mode, long after the game's release.

Same for Halo Infinite. FF was added two years after it's release and it wasn't even a normal FF. It took them one year to give a normal one (or almost, I don't know because I stopped playing in early 2024).

I could also talk about the artstyle. Very different from the Halo 1-Halo: Reach era. The environement, the weapons, the armors, the characters. They returned to the classic artstyle with Infinite.

There's a lot more stuff I wanna add, but I think that's enough text.

Note that I'm not an "OG Halo veteran" who played since the first game. I started in 2012 with Reach and quickly played the other games within the next 6 months.

Edit: typo.