but I was wondering if it’s genuine

Nothing extraordinary, looks genuine. These white ones are OEM drives.

Also, I read on the web that is more noisy that the average hard drive, so I was expecting to hear a noisy motor, but actually the motor itself is very quiet, I think quieter than the other drives I own, but the drive it’s very noisy when it read or write data (it looks like there is a hamster wheel inside). Is it normal?

Motors are mostly silent, yes. It is normal.

Also, not on this unit, but on the other one that I’m testing right now, I noticed a small dent on the enclosure.

If the dent was caused by pressure that’s fine. If the dent was caused by an impact it’s not fine.

as far it works properly I should be fine?

if you crack your head but then you work properly, it’s fine-ish, I guess?

It’s probably the cheapest 20TB Seagate, with what it could be replaced, with something more expensive (not that there would be much difference)?

Small dents in the enclosure don’t affect performance, in fact I received a new drive with one and it failed for unrelated reasons. Seagate honored the RMA even with the dent.

I’d check the s/n on seagate’s site but it might be a white label drive (aka a OEM/rejected drive)