Somewhere after the launch of iOS 11, Apple tweaked Spotlight search to be way more useful. Now, when you search for a person, you can trigger a sub-search that lets you find everything you have on them, from emails, to iMessages, to their contact details, through WhatsApp messages, to calendar events. Anywhere that your selected contact exists on your iPhone or iPad will show up in the list.
And then, you can narrow the results with a sub search.