Deep packet inspection by definition requires the ability to see inside the packet, which if using HTTPS wouldn’t be possible for your ISP.
They can still see the destination IP, return IP, and port number, but that’s it. It would take a ton of storage to log all of that packet data though, and it’d be difficult to come up with a way not to double count it if it’s going through multiple hops on the ISP network.
Logging DNS requests on the DNS server would be a much easier way of collecting that data if they wanted it. I know cloudflare collects aggregate DNS query data through their public DNS server, and Google likely does too.
That’s the thing that gets me about conspiracy theories, they’re always way too interesting to be real. There are real conspiracies out there but they’re almost always boring because the last thing they want is to draw attention.