So, if you really want to learn ROS, I recommend the classes at the Construct (no affiliation): https://www.theconstructsim.com/robotigniteacademy_learnros/... While not “free”, they have been the best I found to date.

That said, as said elsewhere, you should ask why you are trying to learn ROS. For research and academia, it’s great as you get a solid collection of tools/tooling that mostly works out of the box.

However, when looking at creating a product from it... think through the benefits. Much like any framework, even if there is no “cost”, there will always be a real cost in dealing with its complexity. So think through your use case, and design to that.