[edave91]

My ISP provides me with a dynamic IP that changes quite often but is under NAT, so you will never see my real IP, but you will see another IP.  In case someone wanted to breach into my PC, how could they do it? How can NAT be bypassed, and what can I do to protect myself?

[joepa]

They or you probably install malware on your PC. Might be backdoored software (installed by you) or a backdoor installed by hackers after abusing software flaws in the sw you're using.

[Sneed]

My ISP provides me with a dynamic IP that changes quite often but is under NAT, so you will never see my real IP, but you will see another IP.  In case someone wanted to breach into my PC, how could they do it? How can NAT be bypassed, and what can I do to protect myself?

By NAT are you talking about CGNAT? (mobile ISPs do this)
NAT is pretty common across all home networks, but I'm gonna go out on a limb and assume you mean CGNAT where you're all part of a virtual network, and port forwarding is impossible because your connection to the outside Internet is proxied. In this case, look up hole punching or NAT traversal. This is why firewalls are important.
However, if your host machine is compromised, there is no need for an attacker to use these tactics, as you aren't restricted from sending outbound connections which the attacker can use as a tunnel.