You have to connect the two models with one field  over a many2one relation. Then you can automatically use any field of the connected model. As an example, check event.registration that contains the field event_id in a many2one relation with the precision that the model for event_id is event.event. This way, you can from within the event.registration access all fields of the corresponding event.

For more detail please check 

https://www.odoo.com/documentation/15.0/developer/howtos/rdtraining/08_relations.html

Hope this helps.