Sometime programmer name variable like this "mBtnLogin" and sometime "btnLogin" what s does mean ?.

Answer 1

its upto the programmer, they can be named anything.
Its a good pracrice to name them as needed, example to name a variable sum to add stuff, rather than name it 'a'.
btnLogin is a good name for Login Button

Answer 2

I think that the answer you are looking for is that, for better readability, the name of a variable starts with a small letter and all the words after the first one start with a capital letter

Answer 3

when open api classes usually gloab variable name start with "m" so i expected there is some pattern that programmer work on ..