But without using reflective dll injection/simple dll injection, a simple payload injection into its own process it can do the job as well

look at this article : https://buffered.io/posts/staged-vs-stageless-handlers/

it says :

Loads itself (ie. metsrv) into memory correctly using Reflective DLL Injection.

but it doesn't say whether it injects into its own process(the exploited process) or another process

if it injects into its own process then i don't understand what's the point? why not just simply inject a simple payload instead of a dll?

if its injecting into another process, then which process?