Samsung Galaxy Watch 42mm
The latest firmware for Samsung Galaxy Watch 42mm (SM-R815N) is R815NKOU1DVG1, released on August 17, 2022 for 6 regions (CSC). Every official firmware build for SM-R815N, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/R815NKOU1DVG1
The newest build, R815NKOU1DVG1, was released on August 17, 2022. No new build has appeared for 51 months, so regular updates for this model have most likely ended.
Specifications of SM-R815F, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-R540N

SM-R640N

SM-R630N

SM-R420N

SM-R530N

SM-R900N

SM-R910N

SM-R840N
The latest build for SM-R815N, R815NKOU1DVG1, was released on August 17, 2022 (4 years ago).
SM-R815N has had no new firmware for 51 months, so regular updates have most likely ended.
Yes, all firmware files for SM-R815N are 100% official, original, and obtained directly from Samsung servers.
To flash SM-R815N, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing SM-R815N.