how do people get hidden characters like this?

Can you play game on there

My best guess is that they wrote a program that made a string out of the characters they wanted, saved it to a text file, opened it in the editor, and copied and pasted.

CHR$(character code) There are lots of character codes. Those characters start at 57600.

There were programs floating around that let users inject special characters directly into their source code (RMG_IME might be one of them?) but I don't know where they are these days.