This information comes from your end-users browser, and it's not related to login/logout from your platform.

Every time your user is inactive on your website for more than 30 minutes and decides to interact with it again, a new Web Session starts.

How does that relate to the 'Once a session' appear frequency option from my onboarding materials?

During a Web Session, that specific onboarding material will be seen only once; after that, the end-user has to start a new web session to see it again (based on the above rules).