I’ve written a few stories now just at home, however I haven’t seemed to crack one thing. I always write in 1st person as I enjoy the intimacy between protagonist and reader, however what if I wanted to write about an event where the protagonist wasn’t present? How would I do this, due to the story being told from her perspective despite her not witnessing the events?
One more option is to write them as hypotheticals.
"If I had known that..."
If I had known what terrifying events were unfolding while I sat in the cafè! Twenty miles from Pittsburgh, on the other side of the planet, Mary was dialing her sister's number. The phone rang twice, and quickly Susan chirped at the other end of the line. She barely uttered 'hello' when Mary began telling her of a certain fact that would have turned my coffee cold by the mere thought of it.
Blend them in the text by addressing the reader directly. There are ways of presenting them as facts, such that the reader will be dragged even further in the story.
"My dear reader, the thing is that..."
My dear reader, the thing is that as I am telling you of my adventure with the train conductor, Mary, in Pittsburgh, nine times zones away from me, has just lifted the phone receiver and is pressing her thin fingers on the dial pad. Neither you or I know about this, but as you will soon learn, the fact, which Mary is about to tell Susan, will have a quite remarkable effect on our adventure.
Provide them as a vision, and describe the astonishment, later, when you figure out that the vision was true.
For a moment the world faded...
For a moment the world around me seemed to fade. As I plunged in darkness, unable to scream for help, I noticed but a faint silhouette, a shade of milk on a well roasted coffee. It looked like Mary, in her nightgown. She was sitting by her bed, and kept the phone in her lap. She had one ear pressed on the receiver, and with her hand she was quickly dialing. With a chill I opened my eyes. All around me was as I had left it. The coffee was still sitting on the table with its slow curls of steam rising in the air. And the waitress waited, snorting, for the change. Still shaken by that unexpected vision, I fumbled in my pockets and emptied three quarters on the table before dashing outside.
You have several choices.
1) Write a side-story chapter. It could be in third person or first person in the POV of another character that witnessed the event. Third person conveys the detachment from the main character and avoids confusing the casual and distracted reader from confusing the POV holders.
2) Just don't. If the event does not affect the protagonist, it is largely irrelevant. If it is important and when it comes to bite the MC in the proverbial rear guard, the reader will learn about it at the same time as the protagonist. Maybe he discovered letters, a journal (credits to @DPT for this), or someone told him. Opportunity to show how the protagonist reacts to that.
One benefit of #2 is that you gain a lot of time to foreshadow the event's disclosure. It already happened, and ripples begin to hit the MC. Maybe a dragon destroyed a neighbor kingdom. MC finds refugees more and more often, commodities' prices spike, dragonslayer quests are being posted. And then he finds some old guy or a past acquaintance that lost a limb during the dragon's attack. Now the MC is invested in the branching plot. Now the reader is already salivating at the reason for all those strange hints.
You miss on the worldbuilding with postponing however. I am guilty of writing several side chapters on my 1st person web novel.
There is a problem with presenting the side event without the 1st person POV holder being aware, it is almost like a new story. You need to get the reader invested in your side story at a moment it is largely irrelevant to your intimate 1st person story.
On several web novels I just skip these side chapters until they are mentioned on the main plot, a moment when I come back and read them. For most of them I miss nothing by not reading them before they affect the main character (and some times not even after).
So you either write it as a side story in third person or with another 1st person POV, or postpone the presentation of the event until the MC learns about it.
If you are going for the intimate feel, I recommend to keep the immersion and let the reader find out with the MC but either way works.