I would highly recommend posting on somewhere like codementor. They provide the platform but publish your work under creative commons so you remain the owner.

I blogged for years, mainly for solidifying my own knowledge on some topics, but it was always rewarding when someone dropped a comment saying my post had helped them.

Since I started publishing on codementor, so many more people find my posts.

Re your specific questions:

1. Aim for technical walkthroughs that assume limited/no prior knowledge. Find a small project that uses the theory, and then show step by step how to build it, explaining each step in as much detail as possible. This will fill gaps in your own knowledge and help others who will be missing context that you assume is general knowledge.

2. As others have said, I wouldn't recommend copy/pasting anything, but I doubt any academic is going to come after you for plagiarising/popularising their work if you adapt and reference appropriately.