The disk drives are all actually little computers, but the Atari drives -- 810, 1050, XF551 -- are all hardcoded. They have a simple set of commands and you can't override or add new ones from the computer. This makes it easy to completely emulate them without the computer being able to tell the difference. Some of the third-party drives or modded drives are different, you can actually load and run programs on them. That's how the Happy copy software works, for instance. No way to handle that without actually running the second computer in the drive, which is what the full drive emulator in Altirra does.
By default, Altirra's standard drive emulation is set to Generic profile, which emulates a super drive that is a superset of what most drives can do with regular commands. This supports reading, writing, and formatting of all disk formats and a US Doubler compatible high speed mode. You can change the emulation profile by the drop down at the bottom of the Disk Drives dialog to select a more specific drive to target. For instance, if you choose 810, you'll get 810 drive sounds and speeds, and it won't read anything but a single density (90K) disk. Select 1050, and you get 1050 sounds and timings, and support for enhanced and double density, but no disks bigger than that. And so on. Typically one of the generic modes is best unless you're running a specific DOS made for certain disk drives or testing specific high-speed modes.
The XF551 specifically is not particularly interesting to emulate in full mode unless you're testing a custom modded firmware like HyperXF. In stock configuration it's a pretty plain drive supporting SD/ED/DD/DDSD, with no special commands for scanning tracks or doing custom formatting. In fact, the only real thing you get from emulating the actual firmware is accurate emulation of a super annoying bug of the actual hardware where the XF551 refuses to switch densities properly unless you boot off of a double density disk. I've opted not to emulate that behavior in the standard drive emulator's XF551 profile because it's such a pain.