Quite simple.Every captcha has an ID,the bot takes the ID of the captcha you get when you login (automatically detects it I guess) and upload it to 2captcha server (just an example of service).Next,those services also have an earning program (People solve captchas for money $0.4-$0.8 per 1000 I guess? Don't ask me why they do that for pennies -rip these poor souls) so in short,your captcha is solved by someone else using the captcha ID)
Regards
Edit: If you are interested in technical details:
hereThe ID is actually just the domain name of the site. When a captcha is solved they issue a token that is valid on that site for 2 minutes and then that can be passed out to a bot that is waiting for one to use.
Google could easily make some simple changes to the way reCapture works to make it far more difficult for bots. There is a remote IP field in the token but Google doesn't bother to check it.
As for the poor souls who do it, in some third world countries earning 25 to 50 cents an hour solving puzzles is an attractive alternative to being an agricultural labourer.