How do I find a post again?

Bookmark it — bookmarks are collected on your own profile, under Bookmarked, which is the reliable way back to something.

Your profile also lists everything you have liked, every post you have written, and every comment you have left, so a post you interacted with is usually findable even if you did not bookmark it.

Liking is a signal to the author — it feeds their contribution score — while bookmarking is private and just for you.