Will door sensors work on a detached garage and out buildings?

Well, if they are within range of the base unit, I don't see why not.  Here is the link to SimpliSafe's official answer:

https://support.simplisafe.com/hc/en-us/articles/360029374272-What-is-the-range-of-the-wireless-sensors-I-live-in-a-large-home-will-they-work-for-me-

I have a storage shed in the corner of my yard...probably 200 feet from my base station.  Works great, but once I parked a small cargo trailer that was inline between the base station and the sensor.  That led to a connection problem, but once I moved the trailer everything worked OK again.

WiFi is the primary connection between the Base Station and the Call Center, plus any updates to the Base Station are done through WiFi.  If you lose the WiFi then calls made to the Call Center go though a cell connection in the Base Station.  My Base uses Verizon for a carrier.  If you have good cell service where you are then there shouldn't be a problem.

I installed an expanded SS3 system in my daughter's 1906 built house they just moved into and did a window and door sensor in their detached garage. Works fine. Today just added a garage door sensor using a door hinge and entry sensor I saw on youtube.

The steel siding may be a problem.  Steel is good at "stealing" signal strength.