There are clear rules and patterns you follow. It’s kinda like a script with some improvisation in between.
As a relationship anarchist, I couldn't disagree more and it's a regular source of anxiety and tension for me because many people think this, yet differ so widely in what that script looks like. The classic low stakes example of this is when and where and how the first kiss should happen. The reality, though, is that it goes so much deeper. The unspoken rules can extend to things as simple as 'my partner should open doors for me' to 'unless my partner says otherwise we will spend every birthday and important holiday together' to 'my partner cannot be emotionally close with a members of the genders they are attracted to' and so much more. People often don't even consciously know what their needs and assumptions are and it can lead to fights over acceptable behavior or a waxing/waning of sexual and romantic interest. I've seen countless relationships fall apart over unwritten rules that neither side realized were rules or boundaries they wanted in the relationship!