I want to grow a community but I don't know where to start. What are the best sites in 2020 to post articles about programming, math, etc. I really appreciate any help(tips, sites, etc) you can give.
A "community" needs a place to congregate and communicate --- like Hacker News --- which you have obviously already discovered.
Build a web site to host and dispense your content and post links to it here. There aren't many web sites tbat allow you to do this as freely and openly as here. Or you could build your own alternative. Take a look at dev.lemmy.ml