Thats… not true? Go to any Wikipedia page and click on the links and you will see that while the majority of the internet is not admissable, there is still absolutely a serious amount.
RS include articles, papers, books, and reports, as well as other things, basically anything as long as it has a publisher or title it is published under who does some level of taking responsibility for truthiness of the content. An example of this is the generic article for Sleep, the niche generic article for Trees in Mythology, and the specific article for the 2022 USA Inflation Reduction Act.
Wikipedia itself does not do fact checking, because they are focused on making an encyclopedia. They aggregate what mainstream sources can say, and cite those mainstream sources, such that if any claim on Wikipedia turns out to be wrong, you can see exactly where the falsehood originated.
The Sleep article cites many different sources. Some examples: The NIH (National Institute of Health), the medical journal “Sleep Medicine Reviews”, and conferences at The Science Network.
The Trees article cites other encyclopediae, a few books, and the University of Hawaii.
The IRA article cites The Washington Post (a reputable news source), USA Today (An online newspaper), and Politico (A political website and news source).
Now, all of these topics are fundamentally different, which is why you see a variance in the type of source most appropriate to support claims made in any one article. Regardless of the type of article, Primary Sources and Original Research (such as tweets) would not be admissable.
Finally, for adjacent articles to Defold, the types of source most heavily relied upon is gaming industry news sources, gamedev reporting, and gaming journals. Getting Defold in 80.lv and other bigger names in gaming as well as more general mainstream news outlets would be your best bet for supporting a claim to WP:Notability.
I want to be 100% clear though: you can still cite some primary sources for specific things in Defold, where it would be unreasonable to expect them to be cited elsewhere, but as per this Godot talk page thread, it looks like 10 total secondary sources about varying aspects of Defold from various gaming and gamedev and generalized news outlets would be an example threshold of WP:Notability. Primary sources don’t contribute to WP:N but can still be (and often are) used to support claims made in-article. Secondary sources are preferable for both ends.
UGC is wholly inadmissable.