I found two scenarios where this situation is possible. So here is my analysis on them:
- Scenario I - ContentTypeHub
SharePoint online has a hidden site collection named "contenttypehub" (URL - /teams/contentTypeHub). You can host your content types here. They will be available in all the site collections.
- Scenario II- SharePoint Group Sites
If you have created Office365 groups, you will have a separate site collection of each group created where group members can share & collaborate.
Please refer below for more details on SharePoint groups: