How to manually add UEFI boot img to EFI partition

[Moltke]

Not really... I already installed it and I don't exactly want to go back and overwrite it.

So here's something interesting. Before I had Windows 7, and it was on legacy BIOS. So I turned on UEFI... I think I turned off my hard drive as the default boot option. So I was picking where to boot each time from my bios boot menu. So anyway I put it back to boot of my HD, and low and behold it booted Linux Lite automatically. Even though my Windows 10 install has been working just fine as long as I picked it from my UEFI boot menu. But I don't think I was seeing Linux Lite in that menu.

So anyway what, do I copy the /boot/EFI/startup.nsh file to my EFI partition? What exactly is going on here? I can edit the available boot option is my UEFI menu from the BIOS right? But OS installers have access to that and they add an option for you automatically?

First, you should have chosen "install Linux Lite alongside Windows" instead of something else, that way the installer would've done all that for you, but  since you chose "something else" without knowing exactly what you had to do; mount the existing efi partition to /boot/efi, you now have two separate uefi entries. What's going on? You tried to do something you'd no idea how to, and now again you want to do something you've no idea how to. My advice, either leave that as it is right now, and hope that neither a Windows or Linux update overwrite it, or reinstall Linux Lite by using 1 of the appropriate methods: let the installer do the hard work, or if you want to do the whole thing for yourself, do it right.