Settle down can have many usages. as you correctly said it can mean 'get married' and can also mean move to and live somewhere. However this does not have to mean permanently because settled down carries a meaning or gets comfortable/used to as well. Someone can settle in somewhere but move on at a later date. As I just explained it can mean to get comfy somewhere, so you may see sentences like 'The fox settled down on the grass and watched for potential prey.' It can also be used to tell people to calm down, for example when a teacher walks into a classroom and all the students are talking. They may say 'OK class settle down now'.

It can mean both, usually settle down will mean "I want to establish myself in a specific geographic place". But with that, it is usually assumed that the person also wants to find a steady romantic partner, and get married.