Latex

Needless to say that LaTeX is the best language so far to write mathematical formula. A user-friendly introduction can be found online called LaTeX for Complete Novices, which can be read for free. After having gained a pretty good general idea of how to use Latex, other online resource might come in handy in everyday life working with LaTeX, I used to use the famous Latex Companion as a dictionary, but nowadays I found GPT4 to be most helpful.

Also I like to edit Latex file on some local editor, such as WinEDT. The best online editor is Overleaf.

Obsidian + Markdown

Obsidian can be installed online. Make sure you have installed the Latexsuit plugin. There you can define your own snippet.