Sure thing, the bit after the semicolon is a comment, hence it’s not included in the code-formatted part of the term inside parentheses.
Not easy to spot, but the difference is there 
If you look here (which is in the docs), you see the yellow boxes with something like PB6 in it - this denotes GPIO-port B (GPIOB) and 6 is the bit number of that pin on that port - but ->IDR (input data register) is some insight we need to keep up our sleeves only for special users
Yes, interrups do (obviously) interrupt the normal prigram flow, unless interrupts are masked/deactivated.
A brief sum-up of volatile: Need Interrupt Help
I guess the digit select (which @peekay123 pointed out) is not part of the serial data you try to parse, but might be a dedicated line/pin.
What was your error message when first trying uint8_t? And how did your code look then?