In practise I don't remember this being a big deal. At some point I remember helping my dad upgrade us from RISC OS 3.something to 3.11, by replacing the ROM chips, but patches to the OS loaded into RAM were unusual.
The OS in ROM was 2MiB, and looking at some module files intended for potential loading at boot time I have in an emulator, they are around 5-40kiB.
The computers typically had 2 or 4MiB RAM, so there isn't space to replace a significant amount of the OS anyway. (1MiB or 8MiB was possible, but unusual.)
Wow, that's a large ROM. Even in their latest, 68030 based models, Atari never shipped a ROM bigger than I think 512kB (in the TT).
If you wanted the full multitasking modern version of the OS (MultiTOS), you loaded parts of it from disk. Or ran SysV Unix (or NetBSD or Linux, later).
Then again, binary sizes would be smaller on a CISC 68k machine I'd expect.
Some of it's shown later in the long video I posted if you want to look, but the ROM includes the multitasking OS, GUI, BASIC interpreter, text editor, graphics editor, vector drawing editor, calculator, clock/alarm clock, font manager and four fonts, network stack (though not IP), RAM filing system, audio support.
