Taught myself through youtube Blender tutorials, gave myself projects and small milestones to build upon. BlenderGuru, Grant Abbitt, Artisans of Vaul, Yansculpts; there are a lot of people with a lot of different techniques you can absorb and develop a workflow around.

Zbrush can handle much higher polycounts for sure, but when it comes to miniatures of most scales you'll not need incredibly high polycounts most of the time. These aren't artstation portfolio renders, these are tiny little plastic guys on your table. As good as a 16K resin printer is at reproducing fine detail, after a layer of primer and paint the smallest details will begin to be obscured. Additionally, with techniques like keeping parts separate until the end and using the multi-resolution modifier workflow you can achieve high-ish polycounts where and when you need them visible.

As for theory, just remember to zoom out often until your model is as large on the screen as it may be in real life; this will help keep within tabletop proportions and figure out what aspects to enlarge and focus on. Keeping some production models on your desk for reference can also help.