I've had my nano for about a year and I've made many transactions. I'm by no means an expert, but I'll chime in with my limited understanding.

Q: How many cryptos per account?
A: One

Q: Is the account tied to one PC/smartphone device or shared?
A: Account is tied to the Nano. Access the Nano using Live Ledger on any computer.

Q: Why do I need an account?, Why can't I just send crypto to the device?
A: I believe this has to do with the blockchain. Your account has a unique identifier.

I have the Ledger S, but should be the same process as the X. Your ledger holds many cryptos. You only have to create one account by writing down the seed phrase and connecting to your computer & phone.

I do know people who have multiple ledgers that can merge onto their ledger live app. But to keep it simple, follow the directions/video that comes with it. It is super confusing the first few times, but it doesl get easier.

When I transfer, I have my ledger plugged into my laptop, my Ledger Live open on my laptop and have an exchange tab open to copy/paste the send/receive address.

I learned most everything about crypto from Zen God. He does have a paid course, but has tons of free content on his YouTube. I started watching his videos from bottom up. https://www.youtube.com/c/ZenGod1/videos

on youtube there’s a video by Adam Venture on Nano x beginners guide.