College overtime rules are that the ball is placed on the 25 and then you play with “normal” rules in attempting to score. A play clock is used but the game clock is not (I think). Both teams will have an opportunity to play on offense. The winner of the coin toss gets to decide if they want to start on offense or defense. Traditionally, the winner of the toss chooses defense. If there is a tie after both teams have had the ball, they do it again. From the third round on (IIRC), after a team scores a touchdown they are required to go for a 2 point conversion. The game cannot end in a tie.
NFL rules are much simpler. There is one 15 minute overtime period. The winner of the toss will generally choose to receive. The first one to score wins. If the period ends in a tie, the game is a tie.
Which system do you prefer?
I like college better. There will be no ties. I don’t like sudden death although I believe statistically there isn’t much difference in who wins the coin flip. The only thing I don’t like about the college system is the sometimes ridiculously high scores that result.