If the trick to good SEO is simply adding a plugin, wouldn't everyone do it, then it becomes meaningless for crawlers to differentiate content?
That's pretty much it but it's obviously easier said than done.
Generating backlinks is a skill you can aquire, but devs tend to sneer at it since a) it looks a lot like sales and b) there A LOT of shady practices
By comparison, building backlinks and writing content you can always sink in more time, and derive benefit from that extra time investment. So the plugin stuff is more like foundations. You need it, but it's not what your strategy is.
And "building backlinks" is a lot about outreach. You need the content worth linking to, but there's also things you can do to encourage people to link to content that they already like. Relationships with other sites, encouraging people to tweet/like/reddit/socialwhatever your content, that type of stuff.
Another "technique" is to crawl sites that link out to 404s, or outdated guides. Especially if it's a high value site. Then email them and suggest their users might prefer <insert better widget/site/etc that you own here>.
It ("on site" SEO) has zero bearing on what other websites are doing. SEO is just an umbrella word that also encompasses "off site" SEO like backlink building.
Managing on-page technical factors (which is what a plugin can help you with) is primarily a matter of successfully missing your foot with hundreds of potential bullets.
Getting SEO to "good" still requires good content and link building.