Instead of looking for copy trading, I will advice you to learn how to trade yourself and not only go more than forums that you people will only just discuss about trading, and be careful of scammers.
I just want to know how does these platforms select and rank top-performing traders?
The one I know little about among them is copy trading and being a lead trader. Those that do not know how to trade are the copy traders, which are traders that are copying trading from the lead traders. The lead traders are the professional traders that other traders are copying from them. The organization have their trading history and do not just accept any trader to be a lead trader, they take people that meet their requirements. Example is TradingWagon on Binance that can provide this kind of service, they have all your trading history.
The exchange have your trading history and the ranking is based on return on interest percentage (ROI%) over certain period of time like daily, weekly or monthly. The person that have the highest percentage of ROI will be the first, follow by the next person with the second highest ROI% and follow by the next.
What metrics do they use to evaluate their performance?
Like I said, they use ROI%
and how do these platforms handle risk management for traders and investors?
They do not handle anything for you than you to handle it yourself, to use a low leverage and not using more than the amount you can afford to lose to trade because being a lead trader does not mean 100% perfection, a lead trader can also lose and the leverage both of you used can be different is another thing. What is most important about copy trading is that when the lead trader open a position, it automatically opens the same position for copy traders that are following him.
Do they offer tools like stop-loss orders, margin requirements, or position limits? I am confused.
Yes, you can set your own stop lose, and do other things you can do when trading, just that you trade can automatically be opened. Even you can decide to close your position at anytime if you have gained.
Read the link that you provided very well.