You will have to get someone to help with the chain setup... you may not have to change the chain length, but you WILL have to disconnect the chain on the bike now. It may or may not have a "Master link" in it.

here's an article on chain setup... https://cyclingmagazine.ca/sections/...ain-correctly/

here is a video on large/large chain setup...
once again,,, since you are new to bicycle repair, i advise that you go to a Bicycle shop and have a mechanic do it... if he will let you watch, you can learn how chain sizing is done.

The Small/Small method, with the chain already routed through the derailleurs, gives the best results for me, but is not easy for a beginner... i have nearly 60 years experience in bicycle, ATV, and motorcycle repairs.

As to the rear derailleur.. it is a low quality one, but they do work ok until they wear out. The front derailleur, unless it is Bent or damaged, will also do the job once aligned properly. The Alivio Derailleurs are better quality and will be more durable... Deore Derailleurs are another step better quality above the Alivios.!
Rear Derailleur rollers (also known as: Jockey Wheels, or Pulleys) have some looseness to them even brand new... as they wear, that movement, side to side, increases... a Bicycle mechanic will know the difference between ok and worn out.