What do the red numbers mean
I saw you finished the sudoku (congrats!) but I'll answer this question anyway
They can either mean two things:
The numbers are red to show that 'hey! you wrote these numbers down! you can change them again! unlike the base numbers of the sudoku we gave you!', so you can see which numbers your can change, basically
They're red because they're 'notes'. You can say that a certain square has multiple possibilities (out of the nine numbers, only two can be applied, for example) but you're not sure which one, so you note it down in a little corner (the notes are usually smaller than an actual answer) to review later, and hope that it'll help later on in the puzzle