The PC boot loader lists the partitions that are marked as bootable. In Linux, these are partitions where /boot folder is. Systems always starts from a designated partition on a designated drive. The grub settings there dominate. Generally, the second OS that is installed will always rewrite grub settings. That second installer must be aware of the other OS's boot partition so as to launch it. For me, it always works best to have that second install take notice of the OS and add it to the list. Do that in installation, not later, if possible. It is much more likely to turn out right.
If you install Windows first, then Linux, then the Linux bootloader inserts itself into the startup process first. Installer will generally ask you if you want to take notice of other OS and launch them. Most people understand that. Vice versa, install Windows second, it ignores Linux, replaces the OS boot loader, and you have only Windows. Your case is different b/c your 2 OS are more tolerant, the second one will usually be more aware/generous of other OS.
Which drive? Which partition? The BIOS and drive layout are in charge.
When you look at the drive list in fdisk, you see a star by the bootable partition, one per drive in your case. Well, maybe some drives have none. The BIOS drive ordering determines which drive is found first and labels it sda. That first-found drive has a list of bootable OS partitions. That boot list gets rewritten by every install (each time you run grub install, either in the install or later). You must add each desired OS to the settings inside the second OS so this will work.
At install time in second OS, you have choice on where grub is installed; either in master boot record or a partition. Seems like we always use the mbr now, IDKW.
IMHO this was easier 10 years ago because the booter was not supposed to do as much automatically. Configuration of grub was just a single file list of OS stanzas. You'd edit that, then restart. Every boot would list all the OS. It is still that way in RedHat EL. Seems harder now in Ubuntu to get your brain around it.