Yes, voters choose the candidate when they participate in the primary. But before the primary ever happens there’s a lot that goes on in terms of determining who will run in the primary, and what resources they have to run a viable campaign.

Political junkies talk about the “invisible primary,” which Vox’s Andrew Prokop, in an excellent overview, describes as “the attempts by important elements of each major party — mainly elites and interest groups — to anoint a presidential nominee before the voting even begins. … These insider deliberations take place in private conversations with each other and with the potential candidates, and eventually in public declarations of who they’re choosing to endorse, donate to, or work for.”

Clinton dominated this invisible primary: She locked up the endorsements, the staff, and the funders early. All the way back in 2013, every female Democratic senator — including Warren — signed a letter urging Clinton to run for president. As FiveThirtyEight’s endorsement tracker showed, Clinton even outperformed past vice presidents, like Al Gore, in rolling up party support before the primaries.

https://www.vox.com/policy-and-politics/2017/11/14/16640082/donna-brazile-warren-bernie-sanders-democratic-primary-rigged

Not only did the DNC go out of its way to steer resources toward Clinton, there were leaked emails wherein party officials were brainstorming ways to undermine the Sanders campaign with negative messaging.

https://www.npr.org/sections/thetwo-way/2016/07/23/487179496/leaked-democratic-party-emails-show-members-tried-to-undercut-sanders