In my experience, if you sign up via Roku, or probably any third party like Apple TV etc., you may find it difficult to use the service outside of that ecosystem. But, at least sometimes, you can figure it out.
For example, I originally signed up for Paramount+ via Roku. I couldn't use Paramount+ directly. In this case, they knew my email, but apparently, the Roku-created-for-me P+ account used a randomly generated password. (Which is good because it implies that my password at Roku is probably stored in a one-way-hash, or at least that they don't hand out my Roku password to third parties.)
But I wanted to use P+ outside Roku, so I canceled on Roku, waited for it to end, then went to P+ and did a reset password so I could enter a P+ password of my own and resume my subscription directly. So, I was able to do it, but it was a bit tricky.
Could I have reset my P+ password, gotten direct access, and still be paying through Roku? I don't know - didn't try. I was worried that Roku, paying my subscription, but no longer having the right password for the account at P+ might lead to trouble so I didn't try it.
Based on this experience, I think the simple answer is to sign up directly with content companies.