You have to host it yourself, pay for it to be hosted for you, or know and trust someone with an instance who is willing to give you an account.

Nextcloud is like Lemmy. Nextcloud is just the software, to use it you need a service provider that'll give you an actual instance to sign up on. That provider can be anyone, including yourself.

Nextcloud makes free personal accounts available via a couple partners, here: https://nextcloud.com/sign-up/

It's the same thing, you can host it yourself or let someone host it for you. But you're right with your feeling that they don't really prioritize a slim profile for selfhosters with this system. I had to give it up and move to some more lightweight alternatives for those parts I'm using.