Hi Zhang,
Yes you need to assemble only for testing purpose.
for customer you have to create business user for customer, and whatever Work centre you have developed in solution assign it to those business user who want to test it.
This is same thing the way you are testing your development in test tenant, difference is that you are testing with your technical ID(PDI user id) and your customer will test with business user.
Goto Administration woc select General setting view
click on Business user-> Search the customer and click on Edit->Access right
Goto Workcenter assignment--> and find your workcenter and assign it to your users.
when user will login into the system they can test the whole solution.
(Note: if assemble you solution and in testing customer will find any issue then you have to create a patch, before assembling the solution complete all your testing and get confirmation from your client)
let me know if this help you.
Thanks
sunil