Most machines bought in the past year or two will default to using UEFI to boot the system, the advantage is secureboot, better ways for the OS to interact with the firmware, remote attestation, better handling of HD encryption, and a few other things.
UEFI firmware can pick what to boot in a few ways.
In the most compatible mode the firmware will look on the drive for a GUID partition with the partition type UUID C12A7328-F81F-11D2-BA4B-00A0C93EC93B. (type ef00 in fdisk). This partition is formatted as FAT32 (I'm not sure if this is a strict requirement in the standard but I'm not aware of any real firmwares that will boot anything else, coreboot might though). On this partition the firmware will run /BOOT/EFI/BOOTX64.efi (for x64). This is the bootloader and can do whatever it wants. In fact the linux kernel is an EFI executable and when run it'll boot linux!
The other way is by reading a boot entry from EFI NVRAM. This is how most bootloaders (like the windows loader or grub-efi). This will add an entry to a firmware specific boot menu (usually accessed by something like F12 on boot) and that entry will point to a specific executable on the efi system partition. This way even if somebody clobbers BOOTx64.efi you can still easily boot your system. This method can be quite buggy on some machines. For example the NVRAM is supposed to run some garbage collection to reclaim space after a variable is deleted, but some older firmwares don't and will simply brick if you write too many variables. My own Lenovo X220's firmware won't boot from NVRAM entries at all, and can only load bootx64.efi. Any machine manufactured in the last 4ish years should be fine though.
Most linux distros will install grub (or whatever bootloader they use) in some directory like /fedora/... on the ESP (efi system partition), so you can select whatever copy you like from the EFI boot menu. If you have os-prober installed for a given linux then it's version of grub will probably be able to find windows' bootloader and run that for you from the grub menu.
One quick way to tell if you're booting with EFI is to just boot windows 10 and see if the logo that shows up is the windows logo or the logo of your machine's manufacturer. If it's not the windows logo you're in EFI mode.