There can be an infinite amount of certificates for a single domain.
When you setup a connection to a website you basically get a response back that has been signed with a certificate.
Your Browser / OS has a list of certification authorities that it deems trustworthy.
So when you get the response the browser checks if the certificate was issued by a trusted CA.
Now, if the EU forces browsers to trust their CA they can facilitate a man-in-the-middle attack.
In this instance they will intercept the TLS Handshake and give you back a response that was signed by their certificate. Your Browser deems the certificate valid and sets up a secure tunnel to the EUs Server.
From then on they can forward packets between you and the real website while being able to read everything in cleartext
I used Saleor for my thesis. It’s a headless solution, so you will want to bring your own frontend. Uses GraphQL to query everything from products, accounts, and so on. You can set it up quite easily with Docker.