There are a lot of design similarities between the Apple Watch and the Google Pixel watch. You can slide bands in and out of its proprietary strap mount. The way a camera lens snaps into place is supposed to be mimicked by it. It isn't intuitive at a glance and is over-engineering.
If you plan on sticking with the band you received, this isn't a big deal. You need to master the strap mechanism if you want to change up your look frequently. It is easy to swap bands once you masters it. It's so simple you might feel silly. If you do struggle, you are not alone. At the launch event, several employees and reviewers of the company had trouble changing their straps.
The release button is the most important one. It is not found on the bottom of the case. It is to the right of the strap. If you don't know what you're looking for, you may miss it.
The release button should be pressed with a single finger. The strap should cover the button if you slide it to the right. The strap can be removed.
It is difficult to get the strap on again. It may be tempting to just put it back in place. You can see grooves that need to be lined up.
Put on a strap instead.
Line up the edge of the strap and press the button.
The Pixel Watch didn't come with any accessories. You're stuck with the bands until third parties can catch up. You are not out of luck if you want to use the 20mm straps.
You will have to purchase the leather bands from the search engine. The strap pin tool can be used to fit one of yours instead of the default strap. Dan did it with a strap. This is not an official method, but it is still a way to circumvent it. The first third-party adaptors are hitting the market. For example, this one is just $20
We encourage you to get creative now that you know how to change bands. Wearables are personal tech that allow you to express your style, and the Pixel Watch is a great example. Depending on the band you use, its vibe can change a lot. We don't know if this mechanism will be retained on future Pixel Watches, but for now, you might as well have a good time.