Your four extensions are not, I beieve, even in the standard Asterisk configuration. They have either been defined locally or are associated with some Asterisk GUI add on.
I’m fairly sure that there are no reserved numeric extension numbers in the Asterisk core, although some are defined in the sample configuration, particularly those in features.conf.
they are set up by freepbx and if you use freePBX GUI, simply navigate to the feature codes link, you can see all the extensions used there. as i recall, freePBX has a failsafe mechanism built in so you cannot create an extension that is already used, including already used by features.