Each Reflection Renderer component can set up support for Post Process FX with their unique volume layer masks. While most Post Process FX will work without issues with the Reflection Renderer (including ambient occlusion, color grading, bloom, depth of field etc) some of them, especially those that depend on motion vectors or screen based UVs (such as vignetting) may not work nor look correctly.
Once Post Process FX are enabled in the Planar Reflection Renderer you can set up with which layer is it going to interact. It is not recommended to use the same post processing volume for your main in-game camera and the reflections as this could cause some overlapping of effects, for example if you apply color grading to your scene you may end up applying color grading to the reflection and then a second pass of the same color grading through the main camera. This applies as well to Tonemapping effects, being useful in some cases to use a HDR enabled reflection instead and let the main camera’s tonemapping and bloom filters take care of the rest.
In HDRP you should enable Post Process FX in most cases as they are necessary to handle the high intensity values of lights and adjust the exposure of the scene accordingly. Both in Universal RP and in HDRP the use of Post Processing will add some overhead that does not exist in the Legacy Rendering Pipeline due to the different way that it is handled on each rendering pipeline.