These files (often named MstarUpgrade.bin or similar) are typically shared on technical repair sites. Note that using the wrong firmware can permanently "brick" your device.
Specific firmware files like "LEO40FSAIN FHD Firmware part01.rar" are usually multi-part archives uploaded by third-party technicians to platforms like: Download ONIDA LEO40FSAIN FHD Firmware part01 rar
Many repair guides provide external links in their descriptions. These files (often named MstarUpgrade