Chess.com is a very competitive website with a lot of elite players on it. Many of the greatest blitz players of all time play regularly on this site. So don't be discouraged if you find people significantly stronger than you.
So how does a 640 rapid rating compare? Well on this site, you're considered a "beginner". You're below average rating. You're familiar with all of the rules of chess and you know how the pieces move, you probably know a few basic tactical ideas too like what a pin or a fork is, but you struggle to implement these ideas consistently, and you regularly hang pieces.
If you played at a family gathering or with some stranger on the street, you'd probably win. But against serious competitive chess players, you're not quite there yet.
its all good man. i took 20 years off from chess. started at 600 november 15 and worked up to 1100. john bartholomew has good content also. watching his stuff helped me be more positional /defensive. chessable is also a great resource for all aspects.