Eventually you will be able to enter the interiors of many locations directly from nearby streets. Until then, the only way into a business that has indoor scenes in Google Street View is via the business's Place page.
To access a Place page open Google Maps, and search for a specific business or a business type, for example you could search for "Bike Shops in Seattle, WA". Clicking on the business name on the search results list opens the Place page.
On the Place page, look for an image like this under the small embedded map.
If there is a white arrow on the image, the business has indoor imagery. Notice that the image scrolls to reveal other scenes. When you see an indoor scene, click the image to open Street View inside the business.
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