Hi @Gruber, Christian,
The Graph api will list all available sites in an organization. This will need at least Sites.Read.All permission. You could refer to the document
https://learn.microsoft.com/en-us/graph/api/site-list?view=graph-rest-1.0&tabs=http
And currently there is no such graph api to list all sites the user have access to, and I feel regretful to inform you that it turns out to be a by-design one. It is highly recommended that you could raise a new ticket on customers’ feedback. The problem may well be released in the future. Thanks for your understanding.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.