why do they ring bells during catholic mass

The origin of the bells during the consecration traces back to the Middle Ages when Mass was not said in the vernacular and the words of consecration were spoken in a low voice that was not audible to the entire church. Some churches were also constructed in a manner in which it was difficult for people in the pews to see what was happening. Thus in those times the use of the bells had a very practical purpose of alerting the faithful in attendance that the most important part of the Mass was taking place.

Over time the ringing of the bells became codified in liturgical law and continues as a preferred option to this very day. In our modern world with vernacular Masses, the priest and people facing the host, and better visually enhanced parishes the practical purposes of centuries ago no longer exist.

However the bells remain ringing in many parishes due to their solemn reminder that something special and unique is happening during the words of consecration. It jolts awake daydreamers and awakens everyone's human senses to the reality taking place before us. It is indeed a beautiful tradition and rightly continued into the future.
Can you explain why priests are doing Confirmations?

In the past, only Bishops confirmed. Also, what happened to the slap in Confirmations? Priests could always do Confirmation in case of an emergency, so of its nature, the power to administer the Sacrament was not confined to the Episcopal order.

In practice, it was usually conferred by a Bishop except in the case of an emergency. In recent years, Bishops have been given the authority to delegate ordinary administration of the Sacrament in wider circumstances to make it more readily available. In the Archdiocese of Atlanta, every pastor has permission from the Archbishop to confer Confirmation.

The "slap" (which was a light tap on the face, was to symbolize strength the face adversity) was abolished after the Second Vatican Council because it didn't help convey the true nature of the Sacrament which, rather than being assistance to deal with adversity, is the assistance of the Holy Spirit to live affirmatively the Christian vocation in every circumstance of life.