Hello, and welcome.
Apache 2 is not a built-in application. Instead, it is available as a 3rd party package, installable via
pkg_add(1). The name of the package is
apache-httpd. This was used to differentiate it, historically, from OpenBSD's fork of Apache 1.3 that used to be a built-in application. OpenBSD's Apache 1.3 is still available, as a package:
apache-httpd-openbsd.
If you're unfamiliar with pkg_add(1) and OpenBSD's other third party package management tools, please refer to
FAQ 15.