If someone has an apple account and they join family plan what happens to their account

Subscription to what?

Apple Music -- see the following from Get an Apple Music family subscription - Apple Support

If someone in the family group already has an individual or student subscription to Apple Music, renewal of that subscription is automatically canceled when they join the family group.

iCloud Storage -- see the following from Share an iCloud storage plan with your family - Apple Support

When you share your iCloud storage, any family members using the free 5GB plan are added to your family plan automatically.

If a family member is already paying for their own iCloud storage plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.

Other?

They continue to use their own account; that is the point of Family Sharing -- allowing multiple different accounts to share selected things.