They're not comparable. Plugins is basically a O(1) task: You set them up once and then they're done. If your set isn't set up for technical SEO then crawlers can't read it and you won't rank well. But once you're set up correctly and it's not really possible to put more time into managing plugins and rank better.
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.
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.