in addition to what others have mentioned already, there is also a kind of crude but still somewhat effective way to google cotent on specific sites: just put the site in quotation marks. If you, for example, search for knitting “lemmy.world” then the top results will be (usually) knitting-themed posts and communities ON lemmy.world
It works well enough as a starting point and has the additional benefit of tweaking the google algorythm in favor of lemmy ;) and if there is a theme you’re really interested in, it can’t hurt to check all availiable search methods once 'cause you might get different/more results overall.