It is not necessary to confess all venial sins, because the primary purpose of confession is to confess mortal sins. Yes, mortal sins are the main focus. Venial sins are not the primary object of confession, but confession allows us to confess venial sins and to progress in our Christian life, to purify our souls more. This is the principle.
We must not consciously hide any sin, because that would be wrong. But when we concentrate on the main sins or on venial sins that weigh on our conscience, that can be sufficient. As soon as your conscience tells you about certain venial sins, you simply confess them. It depends on your examination of conscience. You can pray and say to the Lord, or to your guardian angel, to show you your imperfections. Then you can bring them to the Lord, purify your soul more, and recognize what you need to confess.