Robotics is complicated, unfortunately, so you can at least rest assured that you're not the only one that finds this difficult or confusing. That said, you may need to add more detail on your particular situation to get useful feedback, but i'll take some guesses and see if it helps.

You have rtabmap working with your camera and now you just want the robot to move around the environment and map it? Is it a ground based robot? if so, maybe the frontier_exploration package is what you're looking for. It will require you to configure your robot to use the nav stack properly and for the depth camera to update the cost maps, but there's nothing inherent to the nav stack that requires the use of a lidar. rtabmap should be able to update cost maps for you (see the Grid/FromDepth option in the docs) if you tell it to.