What kind of Rear derailleur do i have and what RD should I buy as a replacement? I have an old mountain bike (21 speed) and I'm not sure what kind of rear derailleur i have. So far the performance is still okay but I was planning to replace it with my spare RD (SunRun Index) but according to the mechanic it's not compatible. Is there a way that I could still use my spare RD on my bike? if not then, what rear derailleur should i look for? (preferably shimano)
Still a newbie when it comes to bike parts
I think you will need a claw hanger adapter.  The old derailleur has one that appears to be non-removable, and the new one has a 10mm bolt that expects to find a matching hole in the frame, which yours won't have.
You probably want something that looks like this:

There is a sunrace branded one, but it looks the same in the critical dimensions as a generic pictured above.