Circuit board assembly can proceed in different ways. If you are going to do it in a lab, then you would need to design the circuit on a computer, print the design on a transparent sheet, Place this on top of the board, expose it to UV light, and soak the board in a chemical solution to strip off the copper where it is not needed.
It’s a long and laborious process that tends to yield imprecise results. If you want professional quality PCBs, then you should send your designs to an actual manufacturer. They can run your circuit in their factories and send you the items within a few days. You can order small volumes for prototypes or large volumes for mass production.
Professional assembly involves automated machinery with excellent precision, durability, and quality. The service is actually affordable and even hobbyists use this approach.