How to get data via relationships with APIv4?
Not sure if this is what you're after but works for me; the 'join on' is a bit hidden in the 'Select' box.

CRM.api4('Relationship', 'get', {
  select: ["contact_a.display_name"],
  where: [["relationship_type_id", "=", 5]],
  limit: 25
}).then(function(relationships) {
  // do something with relationships array
}, function(failure) {
  // handle failure
})