JK I'm Exploring’s Reviews > This Is How You Lose the Time War > Status Update

I'm like 1/4 through and I have so many basic questions... it feels like I picked up the second book in a series and missed the world building. Or maybe it's just all going over my head.
1. How do they manage to write these letters in a time/place the other will see? Out of all time??
2. What even is the war? All I know is they're on opposite sides.