or maybe nobody has posted that question before (which I find unlikely).

It's very likely. You're comparing gargantuan and incredibly long-established site with a burgeoning, comparatively unknown niche network. I would assume it hasn't been asked yet on this platform rather than the opposite.

First would be to be in a big instance, to ensure maximum reach. Iirc lemmy.world is one of the biggest, so that is ticked already.

Second would be to have such contents posted to your instance or to instances yours follows. The fediverse is generally young, so by simple age disparity, Reddit generally has more info. Going in this line, no matter how niche something may seem, if it was helpful for you, it could be helpful for others, so I'd suggest posting as these come.

Third, Lemmy might be a bit self-isolated, given its purpose is near exclusively to be a Reddit alternative. Considering to use hybrid instances, that bring together community-based posts (in style of e.g. Reddit, PHP forums, etc.), and user-centric posts (in style of e.g. Twitter, Tumblr, etc.) could give you a bigger reach, such as Friendica and Mbin, though for an immediate use, most of those instances are still rather small.

Fourth, RSS bots could help. There's some in the community-based instances, and some on the microblogging platforms (the Twitter and related ones). Posts received through RSS also generally have the same amount of information from title and blurb as search results. Also bridging technologies to be able to simultaneously post to AT Protocol platforms and platforms using other protocols could also help.

Fifth, going in line with the first and third points, hoarding communities and people to follow ensures posts come to your instance. Though as that can easily bloat your feed, I'd suggest checking your instance's rules to see if they allow "mule" accounts, so it can follow everything instead of your main one.

Sixth, if you find a post from another instance and it is compatible with Lemmy, you can search for the link in your instance's search bar. If the post hadn't been federated yet, it should then federate when you search for it.

In short, if you want it to be a search engine/knowledge database, first people need to build towards that. And the fediverse being rather young requires persistence, specially in an age people are trained to have everything immediate.

Trying to think if I missed something, maybe other than "most big sites have RSS or RSS bridges, Reddit included".

You may be better off asking the question here yourself, as the redditor's of yore once did.

I have that problem tbh, site wide search doesn't work very well for posts. If it's a question, go to the two or three most relevant comms (e.g aaklemmy or tech support) and use the community wide search