Also, most of the boards on the market is directly dangerous, and is clearly designed by people that know nothing about electronics.
I tend to agree with you.
And recommending that an opto-Isolator be used in 120V switching situation in a solution that includes a mechanical relay is damn close to creating a Rube Goldberg device. http://www.rubegoldberg.com/
You do not need to do ANY optical isolation if you have a mechanical relay in a solution.
This image attached is *all* you need in a circuit that wants to eventually control a 120V AC lamp. You just need to make sure that your RELAY CONTACTS are rated for more than your nominal AC load. IE; use a relay with switch contacts rated for 3-5 amps at 250V and you are done. Your AC voltage is completely isolated from the low voltage side.
If you want to start talking about optical Isolation and 120V AC circuits, the logical solution is to use a Solid State Relay. A commonly available, and sometimes quite affordable pre-assembled solution for switching AC that appears to the low voltage (arduino side) as a simple LED.
...and if you are confused by AC circuits in general, best to avoid them altogether until you gain that knowledge.