Many churches in Italy are built on older worship sites. What makes the Church of Santa Maria presso San Satiro in Milan different is that the old church was incorporated into the new one, both in design and name.
The original church on this site was dedicated to San Sitiro (Saint Satyrus), built in the 9th century. In the late 15th century, the church was also dedicated to Mary. The name "Church of Santa Maria presso San Satiro" indicates that the new church was "staying with" (presso) the old one.
When the church got its additional dedication, it also got a bit of a redesign. The artist Bramante played a role in the renovation. One of the most interesting pieces of artwork at the church is Bramante's wonderful trompe l'oeil behind the altar; it looks like there's a series of columns that recedes into the distance, but it's just paint.
The Church of Santa Maria presso San Satiro is near Milan's Duomo, and it's open Monday-Saturday from 9:30am-5:30pm, and on Sundays from 2pm-5:30pm. Admission is free.