As AlmaLinux is binary-compatible with RHEL, you should be able to use the official Icinga 2 packages for RHEL on it. This information is also posted here: Icinga » Subscriptions » Support Matrix
In general, the check plugins should be independent of the Linux distribution or even operating system they are running on. There are, of course, exceptions, e.g., running check_apt on AlmaLinux does not really make sense.
For starters, follow the Icinga 2 RHEL installation guide, at least until the “Set up Check Plugins” section, as the later sections are more about an Icinga 2 monitoring server.
Afterwards, follow the documentation further. Maybe take a look at Agent Monitoring. Please take in mind that most information is OS-independent.
The checks aren’t really part of Icinga2 itself as every check is a independent script/program. A lot of the common checks come packaged by the distribution and are preconfigured via the Icinga Template Library.