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So for creating and uploading I took the firmware I found online from LPomykal and uploaded that. I was scared I would mess up the firmware and not be able to recover it. I just got a Linear plus last night and the auto bed leveling wouldn't work, which brought me to the marlin firmware.

I have since uploaded marlin and it seems to be working well. But, in my experience, I'm not happy with just uploading some else's firmware. I want to know how it works so I can play around and mess with it. So for creating my own firmware, all I need is the Marlin.ino file and the config.h and adv_config.h files correct?

When I see a youtube video from someone who is uploading marlin they have 30 .h tabs open and mine never show up. And it's never straightforward, at least to what I have found accessible to download for marlin on Github.

Can I take a Marlin.ino file from an older marlin version, and then add the two .h files?

Also, I was browsing the marlin code, and for a custom boot screen, how would I go about that? I know I have to uncomment a bit, but what must be done for the bitmap file?