I currently have two clapped out Motiv mountain/comfort bikes from Costco in my garage. They're about ten years old, left out in the rain, cheap components.
My daughter and son in law asked if I could fix them. The reality is that they could be replaced for what it would cost to repair them with new parts.
I put an old chain I had on one of them, spayed some WD-40 in the shifters and aired the tires. adjusted a few things. It's rideable. That's about it, though.