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The flashing of Android devices can differ depending on the manufacturer.

For Nexus and Pixel devices, I can refer you to this URL:
https://developers.google.com/android/ota

There is no official way for end users to flash Samsung firmware. There is a tool that is widely used, but actually only repair centers and Samsung itself are officially allowed to flash devices.
https://en.wikipedia.org/wiki/Odin_(firmware_flashing_software)

In any case, you should really know what you're doing when you're falshing. In the worst case scenario, you could end up bricking the devices. If manufacturers officially support flashing, we can link to it. If there are unofficial methods, we cannot write instructions here.