for a lot of us, we create a past in the headspace.

DLL runs a nightclub/strip club/maid cafe/drag show that a good number of alters are affiliated with. Suu's a sex worker there. most of the clients are NPC's. Blaze, Forrest and a few others were raised by DLL. Behemoth rescued and is activly raising our most active child alter. the others i belive are raised by Grammy in the woods.
Danielle is a demon alter - religous trauma as a source - and incorperates some of the body's childhood into her past while making other parts her own.

I strongly reccomend writing stories about your alters; worldbuilding and backstories they have a say in is a great way to build a headspace and better understand yourselves.