Selecting records from one table to create a relationship with different table/layout
I am working on a new project where my users need to create observations based on project participants. On a layout based on the observations table, I would like users to select from a pick list of project participants. I would like 3 other fields to be auto-populated after this selection takes place. The observations table is related to the participants table through a join table. I am unable to figure out how to set it up so users would pick a participant, the participant id and observation id then joined on the join table, and the remaining fields from the participant table populated on the observations layout.
I know this is probably Filemaker 101, but I have never had to do a project like this and it's surprisingly challenging to me.