How to use the these mathematical snippets and sectioning commands works?

[TheFermi0n]

I am unable to figure out how this works. Please help me understand these settings. These seems too handy to miss.

Firstly, the handy mathematical snippets

I tried inside a math environment, outside nothing happens.

Also, how does this sectioning commands works?? I tried writing directly, with a backward slash, with @ symbol, nothing shows a suggestion or does anything.

PS. I am using macOS