
Samsung Galaxy Core Plus
The latest firmware for Samsung Galaxy Core Plus (SM-G3502L) is G3502LUBUANH2 with Jelly(Android Bean), released on September 18, 2014 for 1 region (CSC). Every official firmware build for SM-G3502L, 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/G3502LUBUANH2
The newest build, G3502LUBUANH2, was released on September 18, 2014. No new build has appeared for 147 months, so regular updates for this model have most likely ended.
Specifications of SM-G350, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-G3509

SM-G3589W

SM-G357M

SM-G3568V

SM-G355M
The latest build for SM-G3502L, G3502LUBUANH2, was released on September 18, 2014 (12 years ago).
SM-G3502L has had no new firmware for 147 months, so regular updates have most likely ended.
Yes, all firmware files for SM-G3502L are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G3502L, 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-G3502L.