This will give as a ability to trigger to Create Site as a flow event.
If "Item Create on List" Create "SharePoint Site base"
I feel like those stating that this is easy, or possible with existing functionality, whilst technically correct are missing the point of the request.
This feels to me (and the others supporting the request) like pretty basic functionality that should be a part of the Power Automate (I'll say "Flow" for brevity) SharePoint Connector as standard.
The ability (themes / templates / whatever apart) is a basic function within SharePoint that so much organisation relies upon, especially now with the excellent hub infrastructure. Automations need to be put in place to allow the automated or approved creation of sites, with relatively little complication. Any more can be done with the REST functionality in the HTTP request, and hopefully introduced later.
Essentially, Flow should be able to create sites, and perform very basic work on new sites, if the creator of the flow has the authority to do so at the current site level environment. I appreciate that it's the permissions that are probably making this the hardest, but we shouldn't need to resort to external applications to performs work that would appear to be base functionality of the connector.