You can set it as your default. When I was reading quickly, I thought I saw you asking how to use other search engine while keeping Google as your default,. But in that same area it talks about where you can create the search engine, assign the shortcut, etc you can set one of them as your default search if you want. Just click on the pencil and then Make Default.
And just in case it needs repeating as it gets confusing to people…
When you go to Settings → Search engine it will look something like below:
You’ll need to click on Manage search engines and site search. From which point you’ll see info like below appear:
Some had gotten confused here because they don’t have a Add button under Search Engines but instead only have it for Site Search. Essentially Site Search and Search Engine are the same thing. When you click Add under Site Search you would fill things out as indicated in link I provided earlier. Then to make it appear under Search Engine, you have to click the hamburger menu and set it as default search engine, at which point it moves from Site Search to Search Engine. However, it’s not required to do this. Once you create it under Site Search, you’re able to use that search by using the shortcut/bang you created