If a list contains an "Assigned to" column (e.g. a Issue Tracker list); is it possible somehow to grant edit access to list items created by the user and also list items that has been assigned to the user?

Tormod Solem Slupphaug This is possible through Power Automate. Read more here: https://docs.microsoft.com/en-us/sharepoint/dev/business-apps/power-automate/guidance/manage-list-item-file-permissions