16 channel relay help

I'm doing one of those 16 channel relay board and have some questions on wiring. I know the 1-16 (Relay) goes into gipo (arduino) the ground (Relay) goes into ground (arduino). The 5v (relay) goes into the 5volt (arduino) or VIN (arduino)? I've been told that the five volt on the relay side is a power out and you can use it to power your adreno. But I've also been told that you have to supply 5 volts out of the arduino to get the relays to activate.

Depending on which 16-relay board you have.
Some have the buck supply parts fitted, some have empty spaces there.

You can't power the relay board from an Arduino.
16 relay coils could draw 16*75mA= 1.2Amp.
The control pins only need 2mA sink current each.

All 16-channel relay boards with opto couplers have a design fault.
Opto couplers, but no user configurable opto isolation.

I have the SainSmart, which is the most common one.

I have a 12 volt PSU applying power to the relays

Just connect relay VCC/GND to the 5volt pin and ground pin of the Arduino (assuming Uno).
That will also power the Arduino.