when I create a VM with Network Adapter set to NAT, can I consider it a computer on a LAN whose router can be considered the host computer?

Yes. Imagine there's a 'router' in-between your computer and your VM.

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While the computers of the physical LAN, to which the host computer also belongs, can they be considered machines located outside the router and therefore as machines located on the Internet?

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Yah basically. Outer networks relative to internal networks don't necessarily have to be 'The Internet', but yes that is in principle how the Internet operates. Routers forwarding traffic to other routers.

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I think that those are the causes, for which a NAT VM can be reached by the computers of the physical LAN only if port mapping rules are set on the host.

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Correct. You'd have to punch holes in the NAT (and possibly firewalls along the way) to forward traffic from LAN:PORT to NAT:PORT.

For this most folks just use Bridged networking tho. That essentially puts the VM on the same LAN as the host itself, so they'd get their IP from the same source. (i.e. your home or school router, but keep in mind some routers don't want to give 2 IPs to the same physical port, so that's where NAT + Port forwarding would have to be used).

Basically, yes your assumptions are correct.