I would like to know if a motherboard alone in the case together with a ''mounted/connected'' power supply (turned on) is safe?
Q1 - Is it safe to turn on a motherboard (which have all power connections/connectors attached to it) without having no CPU, RAM, GPU
etc. connected while turning it on? Just to check if it's ''working/first test''?
While there is little chance that you will harm something, doing as you suggest will prove nothing. At a minimum, you need the CPU, memory, and GPU installed to see if the system is functioning. Good luck!
No harm done if it's not kept on a steel table or plate when you start it up
If you have nothing on it , I assumed you'll be starting it out of the case and not in it, so, no conducting material should be under the Mobo.... rest is all ok....