From the link you provided, scroll all the way down.
On the bottom there is a section called ‘releases’. Tap on the latest one (the one shown).
Scroll down below the release notes and open the ‘assets’ section.
First entry should be the apk you are looking for.
You’ll need to enable installation from external sources on Android to install those. Sideloading an app not from the playstore can be a risk but I wouldn’t worry too much about it when downloading from the official iNat repository.