The litmus test for someone who knows truly how to draw is to ask them to redraw whatever object they drew but from a different angle or perspective. Most people to learned by measuring and training their eye won't be able to do it.
In order to draw, you need to internalize the 3d shape. Start from basic shapes and learn to draw them in different angles and perspective distortions - surprisingly difficult.
Then increase the complexity. As you draw, you need to not think in 2d, but in 3d and it needs to be more intrinsic than just "translation". You are so into it, you forget that you're drawing on a 2d plane.
When you learn it this way, it gets easier to do follow up tasks such as shading and coloring.
Syrcra is a great resource on YT and expands on what I just summarized: https://www.youtube.com/playlist?list=PLpHAxHY8zvaMOCO6zeYxq...