What exactly are stalkers?

So i assume that there is an actual protoss guy in there, but are stalkers like dragoons? where if a nerazim warrior is on the brink of death, they are revived into a stalker mech?

Yes, there is a nerazim inside. But while dragoon are the coffin for near death khalai so that they can fight and die a second time for Aiur, the nerazim voluntarily put themselves in the stalker to become super stealthy. I assume those nerazin are the centurions who want to be DTs

what are stalkers? they are walking dumpster fires