I learnt by trial and error mostly. I started a new job and they told me I was going to be developing content in Storyline, so I used the content they'd written in PowerPoint and put it into Storyline manually, without importing it, so that I had to figure out how to do things.
If I look at those early resources now I cringe, but by having actual content to create gave me plenty of opportunities to find out what each button/icon/feature did. Over time I gained more confidence to do more complex things which is where videos and this community really became useful as I had a grasp of the basics.