Doesn't Apple support the major standard device categories: NVMe, XHCI, AHCI, and such, like most operating systems do? The challenges are all for hardware that needs a vendor-specific driver instead of conforming to a standard driver interface (which doesn't always exist). Lots of those can be supported with userspace drivers, which can be supplied by third parties instead of needing to be written by Apple.

> Why does Apple need to make the drivers in a walled garden?

For the same reason that Microsoft requires Windows driver signing?

Drivers run with root permissions.

> Why does Apple need to make the drivers in a walled garden?

Isn't that the whole point of the walled garden, that they approve things? How could they aim and realize a walled garden without making things like that have to pass through them?

Macs and PCs are fundamentally different. Their architectures have always been distinct though the Intel Mac era has somewhat blurred the line.

Modern Mac is Macintosh descendants and by contrast PC is IBM PC descendants (their real name is technically PC-clone but because IBM PC don’t exist anymore the clone part have been scrapped).

And with Apple silicon Mac the two is again very different, for example Mac don’t use NVMe, they use just nand (their controller part is integrated in the SoC) and they don’t use UEFI or BIOS, but a combination of Boot ROM, LLB and iBoot

> Why does Apple need to make the drivers in a walled garden?

Because third party drivers usually are utter dogshit. That's how Apple managed to get double the battery life time even in the Intel era over comparable Windows based offerings.