Hi @Damian,
To help you get started, here is a link to a Story map on writing some basic expressions in Arcade: What is Arcade and why use it
To fetch the attributes from a related table, here is a link to a blog post that goes over some common expressions. See section 2: Fetch an attribute from a related record. The blog has sample code to help you get started. You can copy and paste the code, then change the name(s) of the dataset/relationship/field(s) to match your dataset.
However, you can display attributes from related records in pop-ups without the use of Arcade.
When configuring your popup, click +Add Content then Related Records and select your related table.
Hope that helps!