Most sources come from real documents for special events like birth, marriage, census, immigration. There are other sources too, like newspapers and obituaries. Actual documented facts.
The reason I bring this up is because Familysearch, as I call them, or LDS The Ladder Day Saints, the Mormans in Utah, have it all. Yes, all. The documentation, the tree, etc. They lease out images to websites who create indexes (yes, Ancestry sucks at indexing) in return for the index when they are done or more lately, share the index and link to it at whoever is hosting the images at that time.
Many many years ago, the LDS went around the world and made agreements with all the archives and churches, etc. to come in and scan all documents for preservation. Through wars and other crisis they have managed to copy most of the world, and yes they are still working on it today!
Roy spent a lifetime visiting the archives and getting scans done. He spent years making those photos into usable images for viewing the family sources. Today, I can log into their website and view the images online, make copies, cut and crop in minutes. Sometimes I still have to search line by line and book by book through the images, and it makes me think of Roy in a dusty dark archive somewhere across the sea 50 years ago.
Roy would have gotten a kick out of what the LDS site has turned into, with their one world tree. He did a DNA test many years ago, thinking he would learn so much. In return, the report that came pretty much said had ancestors from Europe! With my envolvement and this website, we were able to find people around the world interested in the Chods, and that was Roy's goal... to bring the family together and let them learn their heritage. The One tree does that too. I have made a few friends, messaged some cousins and all because we share an ancestor in the tree.
As I go along doing research, I will be cleaning up the info on familysearch but it is a slow process. I have a huge HUGE tree. If someone asks about a certain family I will try my best to get that info updated first and then carry on back to alphabetically.