please consider adding permissions to the page Wix Editor: Creating Members Only Pages | Help Center | Wix.com otherwise even non site members can follow other site members but you have no information at all about them, so i am not sure it makes sense, wdyt?
then in the relationship collection use data of type reference to the profile collection
when using wix-data insert you should use the profile _id, but when viewing this collection you will see the column which you configured as primary field in the profile collection (currently title)
try creating some profiles which are using the members area login and fill the collections with some sample data