It is an installer.
The game is much larger than the typical .apk limits, so you download the installer, that connects to the MHF site, checks your patreon level, and downloads and installs the appropriate release for your level and device.
Otherwise you'd have to go through the .rpa file nonsense that android launches have for larger games such as a house in the rift.
If you are concerned about Malware, I have run several scans alongside every new launch and found nothing, and since I'm giving the dude my money, I'm reasonably confident that will remain to be the case.