TL;DR use grep -rG <word>

searches for all files recursively in the current directory for files that include <word> inside them

example:

$ echo "Use Markdown to write and format posts." > tmp.txt
$ cat tmp.txt 
Use Markdown to write and format posts.

$ grep -rH "Markdown"
tmp.txt:Use Markdown to write and format posts.

this is naturally case-sensitive. to ignore cases, add the -i option:

$ grep -riH "markdown"
tmp.txt:Use Markdown to write and format posts.

⚠ Beware

when using this command is large directories (with numerous files and sub-directories), beware of getting text-walled. you might see too many results.

therefore it's recommended to limit your search queries to smaller range of directories/words.