Every mounting bracket is different. Some are simple l brackets, some have a rail that tge brackets hang from, etc. Mine is mounted with a z shaped bracket that sits below the unit and holds it off the siding (bolts are in the foundation.
If you just decide to use something like this, no additional bracing is needed. https://www.amazon.com/dp/B00LDU9R2I/?tag=atomicindus08-20
Those are outdoor brackets. Not indoor.