Which components must be connected to test a water damaged 6s?

Hello Niko. The only parts required to test that the phone is booting are:

A. a charged battery

B. a functioning charger port.

You cannot assume that your original water damaged charger port is functional. As for the battery, its never best to rely on the water-damaged part either. However if you have a multimeter and can read above 3.7Volts across that battery leads, you can always try in place of a new battery. Testing with a water damaged battery does increase the chances of a random software crash, which means losing your data. If you goal is data recovery, just stop, order a battery from a reliable supplier, and do not continue testing until you receive the new battery. Ifixit's batteries a generally very reliable.

If you are confident you have sufficiently cleaned your motherboard, you are confident you have a good charger port, and you are confident the battery is charged. Attached the three parts together and plug into a computer. iTunes should detect your iPhone: "name" within a minute. Otherwise, you can assume the charger port/battery is faulty, or you have more cleaning and repairs to perform on the motherboard.

I personally would not add a screen to this system until after checking the phone with iTunes. ALWAYS connect your battery last when assembling a phone, no matter which other parts are included.