How do we make search results show lemmy pages?

You’d have to attract users with useful knowledge. The site is filled with posts being only a link and 0 comments. The domain will be pushed down just by the low total score it will get. It took Reddit perhaps 10 years to get to the top of ranks, and that was with great content

I think Lemmy does set the canonical URLs correctly, always linking to the origin instance as canonical so it should see it as multiple ways to get to the same content.
Ultimately Google should figure it out if done correctly. But the problem remains that the userbase is tiny compared to Reddit, and mostly focused on the same relatively niche topics in the first place so it's just not gonna rank very high unless you search for Linux stuff.
Does that still get registered that way if identical links/posts are cross posted instead of duplicated?
I regularly see duplicate rather than cross post, which is a shame.

Have informative answers here
Have people specifically using site:lemmy.ca in their queries.

I've mentioned this before, but SEO for Lemmy and Mastodon is particularly poor due to repeat content (since the content is continuously mirrored): https://moz.com/learn/seo/duplicate-content

Tldr- it's not you, search engines don't work well with Lemmy

It does set the canonical URL tags however which should help Google know those are equivalent URLs and to not flag it as duplicate.
But that also means instances have to rank on their own content, none of the fediverse mirrored ones so it'll seriously limit how much the fediverse as a whole can compete with Reddit which have all the content at the same place.

Realistically? For mainstream search? In anything like the top-level results that most people bother to read?

Nowadays, you need to pay Google more than the SEO companies do. Either that, or hope that people specifically search for lemmy posts as part of their search request.

Infiltrate a search engine and develop fediverse-aware indexing that boost fedi pages in search results based on engagement and reach.

Searx works for some things

Kagi indexes Lemmy.