Most language apps/teaching uses "correct/proper", generic language, not slang or regional variations. At least not until one is quite far along.
It just would not be useful since it is more limited.
For example where I live in the US, "you all" is not used at all to mean a general "you". It definitely means "all of you". "You all" meaning "you" is regional dialect/use.
And when I was in Europe it definitely was not thought in English, for regular use, classes .
More below: "Y'all is strongly associated with Southern American English, and appears in other English varieties, including African American Vernacular English and South African Indian English."https://en.wikipedia.org/wiki/Y%27all
You can find the tips by clicking on a lessons button/symbol on the language tree web page. When you do this a pop-up appears with choices for "tips" and/or "practice". Then you just need to choose which one you want to do.
If for example clicking on "tips", it will go to the respective "tips" page for that particular lesson. Such as https://www.duolingo.com/skill/cs/To-be-plural/tips-and-notes .
The tips are separate by the lesson not altogether in one place. So if you want to collect them in one place you can do things like copy and paste, and save them somewhere, do screenshots, save the page or something like that.
The above works in a browser. Not sure how or if the app works the same way. But you can use Duo just fine in a browser on a mobile device. It also does not use hearts/lose lives in a browser.
I usually use my laptop, but when I do use a mobile device I don't use the app anyway since I do not like cluttering up memory with apps if I can do something in a browser, so for me, it works out just fine.
As to the difference between "vy jste" and "jste", from what I remember growing up in the Czech Republic just "jste" is more relaxed. (This was a long time ago though, so just do what Duo wants you to.)
It is also helpful finding material for kids whether books, radio or video. It just sort of ends up seeping in what to use when. I would stick to something resembling instructions/correct language use instead of slang though. But both mean "you are". It's just that Czech is just much more flexible when it comes to sentence structure since it is inflected/has cases. That also allows it to be rather more expressive/put emphasis on things than English by including/omitting things or changing word order and still be understandable, up to a point obviously.
Sorry, just curious if this is something to report—I entered "Jste velké holky," and the answer wasn't accepted because I didn't include the pronoun "Vy." Duo says the correct answer is "Vy jste velké holky." is this a matter of formal vs informal?
I was thinking excluding the pronoun was okay because of redundancy and previous examples. Is this not the case? Should I report?