Collinsville is a car city. The main attractions are spread out along IL-159 (the old Route 66 alignment) and the Cahokia Mounds site sits about 3 miles southwest of downtown. There is no practical public transit for visitors.
By car: A rental car is the most practical choice. Parking at Cahokia Mounds is free. The city's streets are easy to navigate — most things of interest are along the IL-159 corridor.
By rideshare: Uber and Lyft are available and are a reasonable option for short hops between attractions, particularly if you are based in St. Louis and making a day trip.
On foot: Downtown Collinsville is walkable within its core, but you will need a car or rideshare to reach Cahokia Mounds or the Brooks Catsup Bottle.