Quick answer
Outlook can show several calendars in one view: check the box next to each one in the calendar list, then turn off Split view to lay them over each other in Overlay Mode. It is a display setting, not a merge, so the calendars stay separate underneath, the combined view only exists on that one device, and nothing gets produced that a person can subscribe to elsewhere. For a calendar that stays combined everywhere, merge the feed links into one link with Calfeed and subscribe to that instead.
Outlook can put more than one calendar on screen at once, side by side or stacked into a single layered grid. That combined view lives inside Outlook, though: add a calendar, check its box, and the choice is only between two ways of looking at calendars that stay separate underneath.
Add every calendar to the list
For a calendar that is not already there, select "Add calendar," then "Subscribe from web," and paste its feed link. A calendar someone shared directly shows up under "Open Shared Calendar" instead, no link needed.
Combine them into one view
Check the box beside every calendar in the list on the left. In classic Outlook, choose View in Overlay Mode from the calendar tab. In new Outlook and Outlook on the web, look for Split view in the Arrange group and turn it off. Either path lays every checked calendar over the others in one grid, each still color-coded by source.
The limit: a view, not a merge
This only exists on the device it is set up on, and it resets if a calendar drops off the list. Nothing gets produced to hand to someone else. No file, no link, no way for a family member to see the same combined view without repeating every step themselves.
One link instead
For a calendar that merges for good, not just on one screen, paste every calendar's feed link into Calfeed's merge tool. It reads each source and republishes one combined link, refreshed hourly, up to 10 feeds free and 25 on Pro. Anyone can subscribe to that one link from Apple, Google, or Outlook, and manual events sit right alongside the merged ones. It is a read-only combined feed, not two-way sync: edits made after subscribing do not travel back to the original calendars. Downstream, Apple checks about once an hour, Google every 12 to 24 hours.