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In general indeed gets its listings by aggregating them from across the web. They also have a lot of people post directly on their site. They also have the Job Spotter app that gives rewards to people to find and upload pictures of "Help Wanted" signs.

https://jobspotter.indeed.com/

I don't know the answer to #1.

Regarding #2, indeed appears to be some kind of job site search engine, a la google, as most links I've clicked on there have sent me to some recruitment/agency/etc. website... but sometimes you find direct links.

When my family business was looking for a bookkeeper, we listed on Indeed directly. It was really inexpensive and very effective; we received many qualified applicants.