How to change the contents of popover panel

Hello everyone,
I’m working to add a button to a header popover and I found already the code where it creates it (row.prop_with_popover(brush, “color”, text=“”, panel=“TOPBAR_PT_gpencil_vertexcolor”)), but I can’t really find where to change the panel and its layout. I tried to look for the panels name with a text search in the source codes but can’t really find it only where it is used, or even if it is defined there is no draw function, only a poll. What can I do to change the panel?

I guess you found TOPBAR_PT_gpencil_vertexcolor in the code, but note that this is a subclass of GreasePencilVertexcolorPanel

And that one you can find in scripts/startup/bl_ui/ with the actual layout code.