An Astronaut, far from a person who might just hop into a space suit and go jetting into space is someone who have had to complete a very vigorous process to get there.
It might look straight forward where they float backward and forward doing space flips on the International Space Station but each person is trained and specialised to carry out a range of tasks. After all if emergency befalls they will be expected to keep a cool head and fix the problem or abandandon station so to speak.
It was an eye opener to read Scott Kellys book, Endurance, where i learned just how much he went through to train and how eventually he did actually become an astronaut. In many cases though these people are not just astronauts. They are pioneers, very brave people. People who make sacrifices so we can push our boundaries forward. People whose actions might not win many medals or get the front page of the press for everything they do. But their actions in many ways can be as brave as a serving military person.
This link i think goes a long way to asking, what exactly does it take to become an astronaut. https://www.space.com/25786-how-to-become-an-astronaut.html