How would you model a situation where a volunteer can work on many events and each event has many volunteers?

I would create a junction object, for example Volunteer Assignment, with two master-detail relationships: one to Volunteer and one to Event. Each assignment record represents one volunteer on one event, and it can hold attributes of that relationship such as role or hours worked.

This lets me build roll-up summaries on both sides, like total hours per volunteer and headcount per event, and it produces clean related lists and reports.

Interviewers want the junction object pattern and awareness that the relationship itself can carry data. Better answers note that the first master-detail determines the junction's primary relationship for look and feel and ownership. A pitfall is suggesting multi-select picklists or text fields to store related IDs.