The update process on macOS and iOS is very different.
Apple does not sign macOS installers/updates individually for each device. This means that there is no requirement that Apple "still signs" these updates for them to work and be installable.
This means that as long as you have an valid upgrade that at some point worked, it will always work in the future (you might have to set your computers clock back if the upgrade file is several years old though).
In general you should be very careful when downloading upgrades from a different source than Apple itself. Never install the upgrade without confirming that the file is identical to the original Apple file. This is commonly done by taking an MD5 checksum of the file, and comparing it to the known good checksum.
You can find the MD5 checksum of a file by running the following command in the Terminal:
where you replace "filename" with the actual name of the file, you want to checksum.
The md5 checksum looks something like this: ecfcaab9ad3d019ee982abdc84eff102 (just an example)