How do apts get listed on streeteasy?

I work for Brown Harris Stevens and our listings are automatically indexed by Streeteasy once every 24 hours (same for Corcoran, Prudential Douglas Elliman, etc). The smaller firms probably need to send their own listings in which is why you may only see them on their own sites.

Hmm... this is a prudential listing ... interesting ...

There's a lag for new listings, even with the major houses. Can be as much as 2-3 days, depending on the feed cycle. Generally under 24 hours on weekdays.

Most real estate firms have a database for all of their listings, and the database usually has an option to send the listings to other sites, such as OLR or StreetEasy.