What's the point of time machine if internal drive fails?

I have an old macbook air, the hdd reports as fine but I started using it heavily lately and installed a ton of programs that run processes so it will die eventually, suddenly or with warning.

So, I decided to make a clone with third party software once in a while so I can access that at least, and clone that clone back to either a new drive or new macbook. I think that will work.

But I also have another usb that would be good for time machine, but from what I read, time machine relies partially on external backups but also uses internal backups, local files, but if the internal drive is screwed will it fully restore if it can't access these internal files?

It's a bit confusing, also third party software offers backups, archive, and sync, but I don't know the differences, advantages of these, so I don't know what to do, and I read about someone cloning a time machine back up as well so that's an option.

Regular cloning on one usb and dedicate one usb to time machine for incremental backups and restore that if there's a crash when I get a new hd, would that be a good solution?

I also made an image file of my whole computer with third party program called that ends with the extension sprarsebundle, but I don't know what to do with that, how can I restore it? will command R on bootup accept that?

I have a tiny hard drive that's almost full, I want to clean up junk files with mac cleaner x, but I'm a bit scared that will mess things up, so I won't do that until I have backups and clones.

I have 3 usb hard drives so I can do lots of redundant backups, so I'm looking for a good strategy, thanks.