Apache modules
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List of modules in official documentation:
https://httpd.apache.org/docs/2.4/mod/
Debian, Ubuntu and derivatives
Use a2enmod and a2dismod to enable or disable apache2 modules. Check their respective man pages.
Use a2query -m to get a list of enabled modules. Check its man page.
Gentoo
Like many things in Gentoo, distinct Apache modules are compiled - or not - according to USE flags.
Check the output of equery uses apache. Adjust your use flags accordingly and re-compile.
Then use the LoadModule directive within /etc/apache2/httpd.conf
And/or edit /etc/conf.d/apache2 and read the documentation at the top and edit the APACHE2_OPTS accordingly.