There are Twitter platforms like http://www.socialbro.com/ or https://manageflitter.com/ that allow you search users using filters such as genre, country, language, bio, age, profile photo...
How can they find users with these conditions? I didn't find on Twitter API documentation any method for that.
For example, if you look for "marketing" on ManageFlitter it returns us 700k users with that word in their bio. How they are getting that amount? Twitter API only returns 1000 users using search/users method.
I found there are Google commands like [site:twitter.com bio:*keyword -inurl:status] which return every user with a keyword in their bio. Are these platforms using something like this?
There isn't a way to do that with the API, which is limited on every endpoint. The only way to access this much Twitter information is via a data provider. Twitter acquired Gnip, who was one of these companies and there are others. You can subscribe to their service and get the entire history of tweets plus other value-added benefit each of them offers. Unlike the API, you'll have to pay, but the trade-off is that you receive better features and service.