Chinese relatives are so confusing as you need to explain a relationship and the right term for that relation will generally show exactly how you are related to them. There is no such thing as "grandparent" or "cousin", just the cousin who is the son of your mother's younger brother who is biǎo xiōng if they are older than you and biǎo dì if they are younger than you.
The web app works like a family tree. Start by choosing yourself. Then choose someone who is related to you. After that, choose someone who is related to that person. Keep going until you reach the person you want to talk about.
You can also select if they are older or younger than you as in some cases this may change the relative name, especially when regarding relatives like cousins. Wherever it is possible, the tool will provide both the formal and the informal options. It will give the names with pinyin too.
To find out what your mother’s (older) brother is called in Chinese, you would have to select You - Mother - Younger brother. It would then tell you that jiù fù is used when you are referring to him (your uncle) in a formal setting, such as in a work situation. However, jiù jiù is used when you are referring to your mother's younger brother in an informal setting, such as if you were talking to a friend about him.
If you want more instructions, you can also see our video What do you call relatives in Chinese? on Youtube.