Studio, on your computer.
Handy is not a slicer, it passes your print request to the Bambu servers which send g-code to your printer. The options for those prints are limited, so it is unclear to me whether that code is generated fresh for each request or if the servers store a pre-sliced file for each possible combination.
I’m no programmer, but this topic and this site have helped me interpret and make trivial changes in some Start and End g-code.