Canadian ETA

Greetings all, i have a few questions about Canadian ETA

1 - I wish to know if with an ETA, i will still be interviewed at the CBSA when entry canada, if yes, what kind of questions will they ask and what documents should carry along.

2 - If at the border am asked how long i am planning to stay and i say 3 weeks, will my visa be for 3 weeks only or will they give me more time.

Thanks