Walks a path and names everything at or below it, one to a line, beginning with the path itself. With nothing to go on it walks from where you are standing, so find on its own is everything under you — and man hier is the map of what the rest of it is.

The paths come first and the expression after them, which is find’s one piece of grammar: find ~ /tmp -name '*.md' walks two places, and find . -name '*.md' writing is the mistake find has a whole sentence about.

Every path it prints is a link. A directory takes the shell there, a note opens, a file you made opens the page that reads it back out of this browser, and the machine’s own files are plain text because they have no page — man cat for those.

Walking

It walks in the order ls lists things, depth first, printing as it goes. A directory it may not read it names and does not go into:

$ find /var
/var
/var/empty
/var/log
find: '/var/log': Permission denied

then it carries on with the rest and answers 1 at the end, because what it could not read is an error rather than an absence. -maxdepth is a promise it keeps while walking rather than a filter afterwards, so it never goes deeper than you said.

Every test you give has to be true; there is no -o.

-exec

Runs a command for each result — a command, not a shell line. The words go straight to the shell, so there is no pipe, no redirection and no second round of expansion inside them. {} stands for the path, and the \; that ends it is written with a backslash because a bare ; is the shell’s own: it would end the find line before find ever saw it.

find ~ -name '*.md' -exec wc -l {} \;

An action replaces the printing, as it does in real find: that line prints what wc prints and nothing else, and -print beside it asks for both. The shell’s ceiling on how much one line may run applies here too — see man sh.

What is not here

-delete, because rm is right there and nothing here is precious enough to need two ways of going. -size and -newer, because a note is measured in reading time and ls -l already says when it changed. -exec ... +, which batches, when there is nothing here big enough to need it. And -o, -not and parentheses: an expression here is a list of things that must all be true.