That depends entirely on what you're investing in — savings accounts, bonds, and diversified stock portfolios all have very different typical returns and risk levels. This calculator doesn't assume any specific asset class.
Compound interest means you earn returns on your previous returns, not just your original contributions. The effect is small early on but accelerates significantly over longer time horizons — this is often called the "hockey stick" curve of compounding.
No — enter a net-of-fees return if you want to account for investment fees, and consider your specific tax situation separately, as it varies enormously by account type and country.