All references to the Italian spoken in Switzerland that I can find online describe it as standard Italian with a few unusual vocabulary words. But right over the border in Italy, people speak Lombard, Piedmontese, or did so until recently.
How did Italian-speakers in Switzerland come to speak standard Italian rather than a local dialect. Did they once have their own dialect that has since died out? And if so, was it close to Lombard, Romansch, or something else?
People in the southern part of Ticino spoke (and many still speak or understand) a variety of Lombard that belongs to the western Lombard and includes the varieties West of the Adda river: Milan, Como, Lecco, Varese, Monza and Brianza, the south side of the Sondrio province, the south side of Ticino, parts of Novara and Ossola (Piedmont) and the northern part of Pavia. The Lombard family is divided in two main varieties, the western Lombard and the eastern Lombard (east of the Adda river, spoken in Bergamo, Brescia and the north part of Cremona and Mantova provinces). These are the two main varieties, but there is the Alpine Lombard as well, to which the language spoken in the northern valleys of Ticino belongs, a category that is still debated and is spoken in Valtellina in Italy, a part of Ossola in Piedmont, the northern valleys of Canton Ticino, the southern part of Cantone Grigioni in Switzerland, and a part of western Trentino. Alpine variety is a more conservative variety of Lombard that kept some characteristics that later disappeared in western and eastern Lombard, and it retains characteristics in common with the Rhaeto-Romance languages, to which Romansch belongs.
Source: L’italiano nelle regioni: Lingua nazionale e identità regionali: La Lombardia by P. Bongrani and S. Morgana