Question on VM NAT routing. Please help!!

So I was wondering if anybody could clear this concept out for me because I am having trouble understanding it.

So say you set up a VM on NAT mode. How is it that the VM can access the internet but cant communicate with any other machine on the Host's network.

So say for example you try to ping a machine on the Host's network, he sends the packet which gets sent as the Host, the packet gets to the other machine and replies (im guessing), but the reply gets to the host machine but doesnt get forwardid to the VM. I am guessing this is the process why it cant ping any other machine on the host's network.

But following this same process, wouldnt any web server reply get to the actual VM either??