A segment in 365 for marketing app is an essential feature which lets you create a list of related contacts based on some criteria( similar to advanced find). Later it is used to target customers in a customer journey.
So when you create a new segment you have an option to create it as static or dynamic. A dynamics segment is what lets you add a query to filter contacts.
However, there is one thing that will bother you which is, it will not let you add a related custom entity to form a query ( click on add group and select union as relationship)
There is a settings available in Marketing app which will let you enable this feature for any entity in your system. To check this, go to Marketing App — > Settings Section:
Under Marketing Settings, click on advanced settings.
Now click on Customer Insights Sync under marketing setting section which will give you the list of entities on the right side with check boxes.
Enable for the entities which you need and click on publish from the header.
You might see the box is grayed out and you will not have option to enable. To enable the box you need to enable the change tracking on that entity. to do that you have to go to that entity definition and enable change tracking check box. https://docs.microsoft.com/en-us/power-platform/admin/enable-change-tracking-control-data-synchronization
Once published give it some half an hour.
Navigate to your query now and you will be able to add related entities to your query just like advanced find.
I hope this helps!