Both 'guys' and 'folks' are used as an informal way to talk about or address people of both sexes. 'Good morning folks' ' 'Hi, guys' 'Folks' is also often used to refer to people from a certain region or area and people of a specified group or kind. 'Folks around here are very friendly.' 'country folk(s)' But 'Guy(s)' is more often used as an informal term for a youth, man or group of men. 'He's a nice guy.' 'There were three good-looking guys on the train.'

JacquiD gave a very good explanation of "folks". You will hear it very rarely. I would never use the term. You might hear it in the "South" (southeastern area of the US) but you will also hear many other things in that area that would confuse a non-native speaker! Like JacquiD said, "guys" can be used generically to indicate a group of people of either sex. Girls will actually use it when addressing a group of their female friends. "Guy" is simply a somewhat slang word for "man" but it is VERY common. I use it all the time. A somewhat similar term to "guy" but for women and girls is "chick". It is very common. Some might think it is a little bit disrespectful but it is not. You just have to use the term carefully. Older women would be referred to as lady or madam. And you would not call a young girl a "chick". One word you may run into (in the US anyway) is dude. It means "man" but is usually only heard used by very young people. But it is quite common.

'guys' was used mostly in the US Midwest. Hearing people say ' hey you guys' reminds me of the 70's show Happy Days, which was set in Milwaukee in the 1950's. Nevertheless as Jacqui said, many people use it for both sexes. Personally, I think it is said for convenience. ' folks' is also somewhat regional (US South, Midwest) and is rather outdated.

In my opinion, they're interchangeable

Thanks for correction.