I don't use notion, but it would be a fun experiment to install a root CA and see the traffic.

It's probably not always this easy. I see many connections on apps using UDP, so who knows how, exactly, they are encoded.

The data may also be "encrypted", similar to how Zoom "encrypted" data. That is to say, the data is encrypted, but the private key is on the same server. So, if you MITM, it looks encrypted - but there's no security.