Is it possible to filter a subgrid (Or some sort of list) based on what a user has selected in a dropdown field? If yes, how can I do this?
There is no easy way to do this.
For previous versions, there were some approaches to achieve this requirement (supported or unsupported way). Later this broke and ended up with community recommended Plugin approach. Read more
Since this is picklist dependent, I would assume the list items are few, probably you can build views based on dropdown values & manipulate the subgrid view based on the form value. Read more
Worst case, you can build a custom HTML webresource to render a grid by querying using web api filters.