FAQ

Palpatine used resources on Exegol and other parts of the unknown region. Exegol and its surrounding system are nearly impossible to find without a Sith Wayfinder. So nobody would stumble across the system. Also the inhabitants of Exegol were Sith loyalists. They were all completely and blindly devoted to the Sith empire. They were who made up the Sith Fleet. It's possible a fair number of clones were used as well.

It wasn't quicksand which they landed on. But some sort of sinking rock formation above a sink hole. The rocks were big enough to form a sort of natural barrier that kept the entirety from falling in to the cavern.

Rey studied it in the sacred Jedi texts. Also, Leia was training Rey. There's nothing to suggest that Vader didn't know how to do it. But giving life force to heal another living being goes against the ideology of the Sith. Vader was more interested in taking lives.

Kylo Ren destroyed his helmet because Snoke ridiculed him for being a "frightened boy in a mask". Ren had no practical use for the mask other than to look cool and scary. He was essentially trying to emulate his grandfather. So in an attempt to prove himself to his master, he destroyed it. He re-forged the mask in this film because he's still conflicted, between the dark side and the light. The mask hides his emotions and given that he's the supreme leader, it makes him more menacing. Also, he destroyed it to please Snoke. At the beginning of this film, we see that Snoke was but a pawn of Palpatine. When Ren learns this, he re-forges his mask in order to become his own person again. He wants to wear the mask and because he's the supreme leader, nobody can tell him not to.

No

It is a Vexis serpent, as mentioned on the Wookiepedia.

Exegol was never named the Sith Homeworld. It was just another planet associated with them.

Like most dark side users, he was likely innocent, but taken and raised by his master, Darth Plagueis to follow the ways of the Sith. Turning him in to a selfish, power hungry monster.

The film takes place in 35 ABY (After the Battle of Yavin). 1 year after "The Force Awakens" and "The Last Jedi".

Mustafar, near Vader's Castle.