I am trying to access one of my PCs ...
This seems to be the wrong web site for this kind of questions, because it is only about company networks, not about home networks.
Maybe Super User is more suitable for your question.
My first question would be why do I have to activate port forwarding if the IP I use to connect must be the PC IPV6?
I don't know what the router setting "port forwarding" means in your router:
Most routers have a Firewall that blocks incoming connections for security reasons unless you enable incoming connections to a certain address and port.
Maybe the setting named "port forwarding" actually means: "Allow incoming connections in the firewall".
Theoretically, a router might also perform "real" port forwarding on IPv6 the same way it is done in IPv4:
I know that this is actually done to forward incoming IPv6 connections to IPv4 devices (an IPv6-to-IPv4 port forwarding).
I noticed the IPV6 number on whatismyip is different then the one my router claims to have ...
Please beware that IPv6 works like "classic" IPv4, not like "modern" IPv4:
Using "modern" IPv4 your home network has a public, global IP address (let's say 184.108.40.206). Each computer in your local network has a "local" IPv4 address (for example 192.168.178.52).
Currently, IPv6 works like IPv4 worked 30 years ago:
Every computer in your local network has an own public, global IPv6 address. And (unlike IPv4) it is even possible that a computer uses a different public, global IPv6 address for every single connection!
If you try to connect to the IPv6 address of the router and the "port forwarding" setting of the router actually means "open firewall", the connection will not work, because the router and the PC have different public IPv6 addresses.
There are also different types of "local" addresses in IPv6. But unlike IPv4, the local addresses are only used for connections inside the local network. Unlike IPv4, local addresses are not used for data that is transferred over the internet.
You don't need to connect to "whatismyip" to get the public IPv6 address of the computer. Just just have to take a look at the IPv6 addresses of the computer you are interested in.
Unlike IPv4, a computer typically has multiple IPv6 addresses for each network card!
The IPv6 address(es) starting with a "2" or a "3" is the public IPv6 address.