The rules of Chilli mean that you will usually find major fits before minor fits, and usually four-four before five-three. When you find a major fit, you must declare it with a fit bid immediately – if you don't, there is no systemic way of setting that suit later in the auction.
On the other hand, a minor fit is usually concealed in the first layer if there is a possibility of finding a five-three major fit in the second layer; if we want, we can show the minor suit fit later. On the other hand, we may not find every five-three minor fit, because there is no way to show a three-card minor, hoping for five in partner's hand.
Once a fit has been declared, there is no way to change the agreed suit, although you can still play in 3NT or an alternative slam. Bidding after a fit bid is discussed in fit auctions.
There are six ways of showing fit in a neutral auction: