Press the left or right arrow keys to take a stroll through different backgrounds and meet some characters. Press the space key to see the ground in different colors to see where each background sprite starts and stops.
See inside to see comments about how I created this. Take a look at another version of this in my "Scrolling Background Advanced - Top Down" and "Scrolling Background Advanced - Platformer" projects. You can check out my video tutorial for scrolling backgrounds on the official Scratch YouTube channel (look for the Advanced Topics: User-Controlled Scrolling Background video) and find a link to written resources related to this topic in the video description. You might also want to see this older demonstration by the SampleProjectsTeam: https://scratch.mit.edu/projects/583769