Even with just HTTPS they can't see anything other than the domain name. With VPN, ISP's only see one domain: the VPN itself.

Every modern VPN that having an app which runs continuously on your device will do DNS over the VPN, too. I'm just saying that, if you configured the DNS yourself, on a Raspberry Pi or something, maybe you didn't change your DNS and maybe its leaking.

You should also make sure your web browser isn't set to use something else like Cloudflare. In Firefox, this is controlled through the "DNS over HTTPS" setting. You want that to be OFF.

A good VPN provider will have a tool on their website to check for DNS and WebRTC leaks. Mullvad VPN has this, for instance.