Site admins look after your Cloud sites, which might be made up of many Jira, Confluence and Bitbucket systems. They are not necessarily application administrators, their responsibility is about granting access to those systems to other people and looking after the overall set. Site admins can make themselves or other people admins at any level.
Jira admins look after the application, primarily global settings, and project admins are limited to the projects in which they are made administrators.
A site admin can give anyone any rights, Jira admins can give other people admin or project rights, and project admins can only grant project admin to other people.
This is a good place to look at the differences between the Jira and Project admins - https://support.atlassian.com/jira-software-cloud/docs/how-do-jira-permissions-work/ although it does not list differences, just links to lists, but as they are separate roles, there's no direct comparison.
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