Unless PS5 is doing something weird or stupid, I would expect updates to mostly be writes. The patch file will include new data and that will be writing over existing data, or much more likely, written to new/least-used cells with with the controller and filesystem keeping track of where the data is and how is all linked. There will be reads too, of course, because you need to ensure what you are writing is written successfully.
The reason some (not all) PS4 game updates result in a temporary spike in disc usage during updating is because after a number of updates, or more likely when the PS4 filesystem detects a certain degree of fragmentation, the filesystem will decide to create a new contiguous version of the game install on the disc. To do this, may meaning around game installs around to create a space big enough. Anybody who has written a defragmentation algorithm will know how convoluted and complex this can be become. You may need to move other game installs around more than once while you reorganise the physical distribution of data on the platter, just so that you can make a big enough space for your freshly updated game.
I very much expect this normal PS4 behaviour to be completely mitigated on PS5, but this would create every weird virtualised disks for PS4 games. Assuming that is how they are stored.
PS5 changed the update method for PS4?
On PS4, it use patch on copy or something. I can't remember the name. Hench the very tedious "copying" step