2nd part of your question first.
Here’s a link to a video that shows how to dis-assemble your phone. I realize that it is not what you’re asking but it is useful as it shows how to dis-assemble your phone to a point where access to the audio jack is possible, i.e. removal of the motherboard
Here’s an image taken from the video that shows the audio jack on the motherboard.
(click on image to enlarge for better viewing)
As it is a SMD (surface mounted device) type component, you’ll need SMD tools and expertise to effect the repair/replacement.
You may be lucky in that the component is only loose on the motherboard and can be resoldered back on.
Only remove/disassemble enough of the phone to gain access to the motherboard. I suggest that you disconnect the battery from the motherboard, as per the video, before doing any other work on the motherboard as you don’t want to create more problems if you accidentally slip while using any tools etc.
As to finding a replacement part if the audio jack is faulty, a quick search online failed to find any, perhaps you may have better luck. Just search online for Moto G5 Plus audio jack parts or perhaps just Moto G5 Plus parts. Alternatively the creators of the video have a website with a phone number to call if you have trouble finding any parts.
If you find doing the repair too daunting a task, contact a reputable, professional mobile phone repair service and ask for a quote. The repair cost may still be cheaper than buying a new phone ;-)
Hopefully this is of some help