Hi there! Inherited permissions deal with object sharing/access, whereas access levels deal with permitted behaviors. For example, you can specify if a role type can delete documents in an access level, that is not something related to an inherited permission (generally speaking).
If someone has inherited perms on a project that you didn't expect to, they likely have access to the program the project is under, so they inherit access to projects within that program.
Permissions work down, not up. So if someone has manage access on a portfolio, they will have access on the programs and projects within it (going down) unless otherwise specified on the program and/or project's sharing. On the other hand (going up), if you remove someone's access to a specific project, it's doesn't mean they won't have access to other projects in the program or the portfolio.
Here is an article that goes into sharing/inherited perms and has relevant links within: https://experienceleague.adobe.com/docs/workfront/using/basics/grant-request-object-permissions/sharing-permissions-on-objects-overview.html?lang=en
There is more to say here but this is generally the divide...lots of documentation on it in experience league.
Please mark this correct if this was helpful to you in order to help others : )