I don't think the power of backlinking from your own websites, even if they are quality, matches what you get from backlinking from websites of other people. However, you do get some power from your own quality websites.
Internal links really helps. You can find page authority of all of your pages and link high authority pages from low authority pages to pass some link juice to those pages.
Internal linking is good (Google likes it, because it makes it easier for visitors to find what they want, and to find things they didn't know they wanted !). For backlinks from other sites you own, yes, if they are relevant, then the help a bit. If you want the most effective backlinks from sites you own, then you need to be clever about it, and make sure there is no "footprint" that the search engines can follow, so you use a different host (with a different C-class), different WordPress Theme, different content, different style etc. so there is nothing in common between the two sites. That will maximize your gains. There used to be internet-marketing methods that promoted "hidden backlinks" from sites you owned, but they didn't tell you to hide them well enough, so the whole subject of making your own backlinks has got a bad reputation, because when it's easy for the search-engines to see that you are trying to cheat them (by making your own backlinks) then they penalize you for it. But if you do it really well, it still works effectively. Chris