Not to be confused with a Customer testing environment, which is your own dedciated space for tests and practice. Public Testing Servers (aka PTS) are intended for cooperating with invited stakeholders on the development of features, and the testing of more complex changes that require external services (e.g. SSO). To facilitate specific goals, data and software versions in PTS often change between sessions and as such they are considered volatile. They are not suitable for everyday customer testing needs.
If you are invited to use these servers, it will be for a specific time period, and you will be given detailed instructions on how to gain access.
If you're interested doing testing or practice of your own, please see the Customer testing environment article for more details.