AncestryDNA and MyHeritage both offer a basic online family tree builder for free, but you can only add relatives you already know about. But if you subscribe to these companies’ genealogy services, their algorithms will find relatives for you.
The company will spot a name and set of dates on your tree that seems to match someone in their database, and ask you to confirm that it’s a match. Once you’ve established that this person is indeed your relative, you’ll receive suggestions for additional people who seem to be related to that person according to its records. And so on and so on.
Over time, your tree grows and grows and grows in this manner.
And if you find living relatives who also have family trees on the same platform, you can get in touch with them (if they give you permission) and share your family trees with each other. That can allow you to find a huge number of new relatives in one fell swoop!