Hi @Arioule ,
Based on the issue that you mentioned, I think you have some misunderstanding on the relationship in CDS Entities.
Currently, within CDS, the Users Entity and the Teams Entity are related together through the "teammembership_association" relationship (Many-to-Many). There is no actual Entity in CDS to store the relationship between the Teams Entity and Users Entity.
More details about the Relationship in CDS, please check the following article:
https://docs.microsoft.com/en-us/powerapps/maker/common-data-service/create-edit-entity-relationships
For your first question, I suppose that you want to display Team Members for specific Team in your Model-Driven app, is it right? Based on the needs that you mentioned, I think the Sub-Grid control in Model-Driven app form could achieve your needs.
On your side, you could consider create a Model-Driven app based on your Teams Entity. Edit the Main Form of your Teams Entity, then add a Sub-Grid component into it as below:



then click "Save"-> "Publish" to publish your changes you made to your Team Entity Main Form. After that, edit your Model-Driven app, enable above Teams Entity Main Form as the Only Edit Main Form. Then publish your Model-Driven app.
When you run your Model-Driven app, and edit one Team record, the related Team Members would be show up in above Subgrid section. Within that section, you could also add new members to this Team.
For your second question, do you want to fire a flow when add or updated event occurs on teammembership Entity? I afraid that there is no way to achieve your needs in PowerApps currently.
If you would like this feature to be added in PowerApps, please consider submit an idea to PowerApps Ideas Forum:
https://powerusers.microsoft.com/t5/Power-Apps-Ideas/idb-p/PowerAppsIdeas
Best regards,