for calendar events: this is a very common use-case with no easy cross-provider solution that I’m aware of. If the other person wants to add/edit events, they need a provider-scoped account.
My own solution to this:
Each person publishes a (private) subscriber link their own calendar (think “family events”), this is a icalendar / webcal link. As calendar apps can show many side-by-side, date collisions can be seen.
This requires the other person to be able to publish as ics (Google uses “Secret address in iCal format” wording). Most email providers with cal functions can do this, ecloud / nextcloud too.
For the files: you can create a nextcloud share folder that persons with no account can upload data to (“Allow upload and editing” → optionally password protect)