MusicTechHelpGuy adds a synth bass patch in Logic Pro and plays a simple line against existing chords, using the built-in MIDI monitor to show exactly what gets recorded.
The monitor displays channel, pitch, and velocity as you play, giving a real-time look at the data behind every note instead of just the sound.
He also clears up a common source of confusion: C3 reads as middle C in most DAWs (the Yamaha standard), while traditional music theory calls that same key C4 (the Roland standard). The pitch never changes, only the label.
After recording, he quantizes the take, trims it into a clean four-bar loop, and repeats it to build out the arrangement.