Now some explanations. Not every line has to continue, but usually when you have another shape where it would, then that shape is somehow related to that hypothetical line. When a line starts/ends it’s quite often either in an empty space or in a spot that has a visual reason for that - another line (at a different angle) ending, continuation of a line further away, being in line with other points etc. Also not all edges have to be in line or parallel, but they should if they are close together and nearly in line or nearly parallel. And obviously there are good looking exceptions to all that, but it works in general.
While mixing different kinds of shapes can work very well, it has to be that - mixing, not putting them next to one another. Especially not in separate areas, because that makes them look like parts of different designs.
Now a disclaimer: I’m not an expert designer nor is it a professional advice, so don’t think of it as some absolute truths 
Those are just things that work for me and seem universal enough. And I’ll repeat - Discord can be more helpful with that kind of stuff, but if you want something from the forums, there is a quite good guide: Tips and Tricks to Automation Car Design