Yes, they can be confusing. I’ll try to get you going.
I don’t have a Baronius, but my 1962 Missal has a liturgical calendar in it. Yours probably does as well. Thursday April 3rd is a ferial day. On ferial days, the propers of the Mass are the same as the previous Sunday. In this case, Low Sunday (first Sunday after Easter.)
Friday, April 4th is the Feast of St. Isadore. You will see this in the section of the Missal called the Proper of the Saints. The Feast of St. Isadore has no propers (Introit, Collect, etc.) unique to his feast day, so the missal directs us to the correct Mass propers for this day. A page number is given to direct us to the section of the missal called the Common of Saints.
In the case of Saint Isadore, we are directed to the Mass: * In medio * of a Doctor. St Isadore was a Bishop, Confessor, and a Doctor of the Church.
Moving on to April 5th in the Proper of Saints, we find St Vincent Ferrer, Confessor. His feast day uses the Mass found in the Common of Saints. Mass: * Os justi* of a Confessor, not a Bishop. Note the exception in your missal. This day, we use the Collect given under St Vincents name instead of the one in the Os justi. Read both and see the slight difference.
Hope this helps