"Please excuse me."
Kudasai is the Keigo (respect form) of くれ which is related to くれる To Give
Please wait (Literally: Please give me your waiting)
Domo has its origin meaning "No matter how, words can't express…". Today we can almost use it like すごくVery/Extremely. It works like an adverb so it often comes at the beginning, as we say (I am) very grateful, (I have been) very impolite. When it is used at the end it is because the "thank you" at the end is implied.
"onegaishimasu / kudasai" are verbs, so they are at the end.
Was it "すごい!" ?
It is an adjective meaning "to a great extent" and it can be dreadful, awful, horrible, wonderful, amazing... Japanese often say it to give an exaggerated response, usually positively. "すごい! ありがとうございます! " would be like "It's awesome! Thank you so much!" (when the listener has helped doing something neatly).
Just a quick and incomplete note:
どうぞ: When you give permission to the listener to do something, as if you are a host. e.g. Please (go inside my home), Please (start eating), etc.
どうも: To intensify a succeeding polite phrase. See above.
ください: To ask the listener to give you something or do something for you. e.g. お茶をください， 読んでください.
One single app can't teach the immense complexities of a language. Duolingo is excellent for practice and the chats also help. I use Duolingo and Obenkyo for practice. Obenkyo for general explaination on grammar, structure, etc, Jisho Dictionary is my favorite dictionary with really good and comprehensive definitions. Speaky to find people to practice with. The best approach is diversify your learning platform.
I'm sure you can hear it used from time to time in Japan. Often it is used for thankfulness.
Just a side note that politeness depends on the context. すみません would be good for daily life occasions, but in specific context (such as a salesperson to a customer), すみません is not polite enough. It is outside the scope of Duolingo though.