The SENCo is just there to help your child, and although they can be of varying quality, the idea is that they work with you and provide advice to you and the setting to support your child.
They are usually just a member of staff who has this role, ie not particularly qualified in anything, and not in a role of diagnosing your child.
My experience in a nursery setting was very hands off from the SENCo at nursery. However there was also a local authority SENCo who came to meetings with nursery staff and observed my child. She seemed very useful at the time: very focused on helpful interventions, good at planning ahead for school transition.
The nursery staff or SENCo might at some point want to refer your child on to a diagnostic service in the NHS, in our case, for autism.
But the real point of SENCos is to ensure that the nursery are providing the right sort of environment for your child to thrive.
And you don't have to have them involved if you don't want to. I would, because the more differentiated the service for my child, the better, in my view. But your call.