Tl;dr

Star gets mad at her mother, screams "It's not a phase mom! Toffee did nothing wrong."

Decides that magic should be destroyed forever because Mewnie's Queen is too good at it and because Marco is dying from a magical poison. So it is destroyed in every universe because Glossaryck said "Star is the main character so what she wants is morally right."

Then he dies because he is a magical creature.

Star realizes that even though the poison no longer exists, there is no way to see Marco again because the fucking portal network is now gone for good...

So she uses the last of magic to realize she's made a horrible mistake and goes back in time to stop herself from destroying magic..

Naw I'm kidding she actually makes Mewnie crash into the Earth the planets begin to merge, causing cataclysms on both planets that kill many in the process....

The show pretends this is a happy ending as we see Star's friends magically survived the merging of the planets thanks to Princess Horsehead (who should be dead as she is a magical creature but consistency is for losers)

RIP: Spider with a top hat, everyone in the literal afterlife, and the millions of people who Star killed in the cataclysm of Mewnie and Earth merging.

Funfact: People who would watch a show like what the first two seasons promised are fans of magic existing more than they are of genocide. So this ending was not well liked and I personally do not consider it canon.

Especially since so many fanworks have Star hanging with Dipper and Luz from Gravity Falls and Owl House who being seekers of the supernatural would never be able to in canon forgive Star for this...