Here's a photo of my HX Stomp with the WIDI Jack (small blue thing, top L corner of the Stomp), a MIDI Baby 3 controller and a Pedalnetics Helauxy Duo. This is how it all works:
The Helauxy Duo is connected to the EXP socket on the Stomp, and gives me Foot Switch 4 & 5 on the Stomp. I use FS5 as my 'scroll down' command (via the WIDI Jack) to OnSong on my iPad. This scrolls the page down, or if it cannot scroll further in that chart, it goes to the beginning of the next song in the OnSong set list.
The WIDI Jack is connected to the Stomp's MIDI OUT. It recieves MIDI info from the Helauxy FS5 > Stomp, and sends via Bluetooth MIDI to OnSong on my iPad.
The MIDI Baby 3 is connected tho the Stomp's MIDI IN. There are three functions - PRESS I use to alternate stompbox mode & snapshot mode, SHORT HOLD I use to alternate play view & scroll preset view, and LONG HOLD I use to go to tuner.
I hope that explains how it all works clearly.
OnSong has lots of MIDI functions to choose from (PC, CC etc) including 128 SONG SELECTs but I've never used that function myself. As I have over 1200 charts in OnSong, 128 isn't enough for me to use in that way. If you can manage with 128 songs or less, that might work for you @Cliff Edge