How do DMs like BLeeM connect things that happened in sess 1 with stuff in sess 50

As a DM, i would really like moments in my campaign where everything suddenly 'clicks' and ive seen BLeeM does it really well. A twist that was foreshadowed in sess 1 and revealed so much late.

As someone who writes my campaigns as they go on, this is nearly impossible to do. I was wondering how DMs do it