Separate from the in-game easter eggs page. This covers the secrets hidden inside the trailers themselves - audio cues, background details, callbacks, etc.
Categories
Audio cues - radio snippets and background dialogue. Background detail - billboard text and signage in the scenery. Callbacks - subtle nods to previous GTA titles.
Each one is tiny and easily overlooked. Frame-by-frame study by the community has revealed most of them.
Documenting your finds
If you spot something the community hasn't already documented, the GTA convention has been to share short clips with timestamps instead of long-form video. Clipping programs such as the GTA 6 clipping tool are made with that use-case in mind - extracting a 6-second beat from a lengthy trailer/playthrough so you can share the discovery without making viewers scrub through the video.