Hello Dansgrandpapa and welcome to these forums. You'll find many helpful people here and a lot of useful information. Make sure that you:
It all depends on what you want to do.
(a) If you add an FX to the Reaper track, then yes, it will be applied to the recorded audio ("post-recording"). That way, you can change the FX, add other FX etc, without having to record the track again.
(b) Or you can add FX as Input FX; they will be applied to the audio input before it is recorded. But that it a bit inflexible Right-click the [IN] button for the menu to get to the Input FX chain.