Question about Factions and Role Playing

I’ve not played the previous games but in prep to start playing stalker 2, I’ve watched a couple of lore videos. And I have some questions…

• Are the four main factions from the previous games in this game also? Any new ones?
• Which factions are always hostile to the player?
• Can you align yourself with a faction?
• Do other stalkers also appear in the world? Are they hostile?
• Is it best to play like… only shoot those who are hostile to you? Or shoot first and ask questions later?
• Or how do you tell friend from foe?
• Can you murder everyone if that’s the way you want to RP?
• Are any important NPCs non-killable?
• If you kill a friendly by accident or on purpose is there a reputation impact or any other consequence?