How do I get my Sims in Sims 4 to interact with the Sims world.

It's basic Sims gameplay. You have to click on the new game button (the "cross" at the top right of the screen). Clicking the cross will take you to CAS (create a sim). There you create your sim(s). After you have finished creating your sim(s) you click on the play button at the bottom right of the CAS screen. The next screen you will see is the Home World Screen. There should be (with the base game), three worlds shown there: Willow Creek, Oasis Springs and Newcrest. If you click on one of those worlds the next screen you see will be that particular world with lots to build on, pre-exsisting homes and all the buildings you are used to seeing. Newcrest by the way is an open world with just empty lots for building. You have to place your sim(s) into a pre-exsisting home or build one on an empty lot. After that you can begin to interact with the other sims living in that world. Have fun.

The Sims 4 is not like the Sims 3. There is no big town to explore. The lots you see are the entire town, but there are multiple "worlds" or towns your Sims can live in. Some of these lots are community lots, like nightclubs, lounges, libraries, or museums. You can edit those as much as you like, also. You have complete control over what is and isn't in your town, and you're able to customize every lot to your liking. What you experienced on the Sims 3 isn't what you'll experience with the Sims 4, but hopefully you can learn to love the new features.