1. use the browsers installed on the local computer (free)
2. use Selenium Grid and browsers installed on remote computers (free)
3. use Sauce Labs (not free)
Options 1 and 2 are limited as for 1, only browsers for one operating system can be used.
For option 2, browsers for various operating systems can be used but the remote computers have to be set up and maintained.
With Sauce Labs, you get access to many operating systems and many browser versions.
Knowledge of Maven is needed for the Sauce Labs option.