This is the first game of the series right?
Kinda...
To be exact the series can be summed up as:
1) Muv Luv(Extra + Unlimited) - this game, the original.
Extra is a very short super crap romance novel that introduces one version of characters in a world resembling our own - no otherworldly mumbo jumbo.. Would be absolute disaster if taken outside of context as it does not stand out much out of any random romance VN.
The characters get drastically changed in later games and this short 'game' is ALMOST fully pointless.. but it's super short, so there is that.
It adds a bit context to some things that will be mentioned in the other games.
Knowing that this world 'exists', knowing characters from this world and that MC of unlimited/alternative is originally from 'this' world is enough knowledge for the later games.
Unlimited - you have to get through Sumiko + Meiya route of Extra to unlock it.
This is what the 'main game' is all about.
It's an 'Alternate Universe' setting with alien species invading.
In this version - MC is super weak 'Extra' game one BUT everyone else is the 'Alternative' version of their selves... with usually minor to big changes of their background, but mostly retaining same personality.
To summ it up the ending of this one is relatively 'bad' for humanity. This is not so much of a spoiler honestly, the interesting thing is how the process goes.
The game itself is pretty weak individually but does give some insight into playing the 'much better games' - 'The Day After' and 'Alternative'
1.1 The Day After - probably the most noteworthy non 'Alternative' game.
It is the direct continuation of 'Unlimited'.
It is a game consisting of '4 games'. 4th has yet to come out as of my post in July 2022, although it has been announced already.
It follows events after 'The Day', which is slight spoilers to how Unlimited ends. So far I would say it's not going as strong as Alternative, BUT it's certainly a 'good' VN. Just don't go expecting 'Alternative' tier story and you will be happy.
2) Muv Luv Alternative - the VN you probably heard the most about and the reason the whole series is popular.
It's a very deep story that is essentially 'what if' MC could relive the 'Unlimited' story while knowing every event that has happened in 'Unlimited' and having a Giga Chad trained body retained.
Since you are a Giga Chad protag in this game with future event insight - you kinda do the things that any Giga Chad would do and change the fate of the world.
Don't let my joke summary drive you off though, it's mostly there to avoid spoiling anything big.
This game is considered one of best VNs of all time for a reason, it's a truly mature story which is not afraid to touch difficult subjects.
2.1 Schwarzesmarken - the arguably 'best' 'big' side game. Just like how you could have concluded from the name - it's 'Germany' perspective in early part of 'Alternative' timeline.
Untranslated to EN.
2.2 Alternative Total Eclipse - yet another big side game of 'Alternative', this time focusing on UN.
Got a recent remaster version.
3) Mostly every other currently released game - those are mostly 'extra stories' with different cast of characters, usually based on one of the other countries perspectives in 'Alternative' timeline. Or some short stories with existing ones... Basically 'side dish' for milking big time fans.
Nothing too noteworthy and better ignored.
4) Muv Luv Integrate - a game which is planned to be a direct sequel to 'Alternative'. Currently very little info is available(july 2022)
TLDR:
Game has 2 timelines.
1) 'The Day' Time line - Extra(mostly unrelated) > Unlimited > The Day After(4 games, last is not released yet, july 2022)
A "what would happen if no 'Alternative' time travel shenanigans happened".
2) 'Alternative' Time line - Extra(mostly unrelated) > Alternative + loads of extra side stories for Alternative > Integrate sequel(not released yet, july 2022)
The Alternative being essentially same story as 'Unlimited' but with MC having prior knowledge of 'Unlimited' events leading to drastically different conclusion and overall drastically changing how events play out.
Nothing as complex as 'Fate' once you break it down into where something belongs.. So don't be afraid of the big amount of games, it's in reality a very small series with the only top tier game being 'Alternative', but it wants you to play some other shorter/worse games to understand a bit context.