This is really more of a general PHP programming question and might get closed for that reason. But before that happens, let’s see if I can’t clear some of this up for you.
Those strings that start with % signs are in “printf format”. For example, %2$s translates to “replace this token with the second parameter, and treat it like a string”.
When you see a variable in curly braces inside a string, like "Today is {$day}", that’s the same as putting the variable there without curly braces, like "Today is $day". But formatting the variable like this makes it stand out, so it’s more clear there’s a variable there. It also prevents other characters next to the variable name from getting interpreted as part of the name. Without curly braces, would PHP know what to do with "Your robot name would be {$firstName}Number5"? It would see "$firstNameNumber5" and look for a variable named $firstNameNumber5 instead of simply $firstName.
I’m not sure what you’re asking about in the last example. Is it the __FILE__ you’re confused by? That’s a PHP Magic Constant that gets replaced with the full path to the file that bit of code is in.