Plone relations are simply awesome (if you don't know what relations are, please see @pbauer 'Why Relations are awesome" talk). There isn't anything as powerful as relations any other open source frameworks out there. Yet, they are some obstacles in using relations in Plone 6 Volto.
RelationList Widgets are not yet available.
If you have custom content types that use relations, you won't be able to populate them using volto forms as there are no widgets available to handle listing relations and selecting them.
Accessing Relations in Volto
Unlike classic templates you can simply select and display relations in your views. I know there is way to create custom web service but we'll need something that is available as an external addon perhaps.
Am posting this topic to drive some conversation and hopeful find someone who bit more skilled than me to remedy these issues so that relations could simple work out of the box in future releases of Plone.
No, so far it only allows getting relations of one item (incoming or outgoing). There you can filter by relationship. Using a second endpoint (@relations-catalog) you can also get all relations of one type.