You use a daw with a piano vst, its a program that has a virtual instrument loaded, you can pay for one or look for free ones that dont sound as good as the paid ones. i personally use cubase artist 10 with keyscape for the piano
Hey bud. Easy!
1. While you're recording yourself playing, make sure you're hooked up to a VST which records the MIDI data of your playing.
2. Convert this MIDI file into audio (.wav) and use a video editor to align the audio with video.
I use PianoTeq, and I love the amount of reactivity and dynamics possible with it.
Here's how the audio turned out in mine: Comptine D'un Autre Ete