If we give something up for Lent - like alcohol - does that apply to the Sundays during Lent too, or are they exceptions?
Many people say that Sundays are feast days and therefore don’t count. My own view is that if I’m giving something up for Lent I mean for it to last from Ash Wednesday through the Easter Vigil.
Sundays are solemnities. I think that the best answer to your question would be in the Canon Law itself:
Canon 1251 Abstinence from eating meat or another food according to the prescriptions of the conference of bishops is to be observed on Fridays throughout the year unless they are solemnities; abstinence and fast are to be observed on Ash Wednesday and on the Friday of the Passion and Death of Our Lord Jesus Christ.
Here’s a link: vatican.va/archive/ENG1104/__P4O.HTM
Yes, this is talking about the abstinence on every Friday in honor of Jesus’ Passion and death, but the same rule applies: a solemnity is a major feast day of the church, where the faithful should be rejoicing. That is also why, during Sundays, it is unlawful to perform any work or activities which take away from the joy of Sundays (also taking from Canon Law, 1247).
I hope you got the answer you were looking for.