A UAT usually happens when the solution is build on behalf of someone. Company A asks Company B, and the UAT is done by A, so that they can test the delivery. If you have multiple layers of requirements, SIT tests the technical req’s and UAT the high level business requirements. YMMV
Often the UAT is performed by company A’s testers or subject matter experts from A. So there are some tricks here:
As always, really, it’s a matte of figuring out what to test and who that has the skills to test it - and then get a collaboration going with them.