How to find out who is behind the process?

Question

CentOS

Hello.
There is a VPS, several system users, LAMP, sites (domain1, domain2, ...). The htop monitor shows some processes and their users.
It's not entirely clear to me why there are processes started by users domain1, domain2 and separately apache, mysql, for example. I imagine that under the domain1 user, apache is launched, which in turn accesses php, and php makes a request to Mysql.
What am I doing wrong and how can I find the original source like when htop shows CPU 100% clogged mysql process with pid=X?