Quick answer
On a Google Pixel, long-press a blank home-screen area, open Widgets and choose an available widget from its app. Drag or place it where the grid has enough space, then resize it if handles are supported. Pixel's built-in At a Glance area is not the same as an ordinary freely movable widget, so don't assume it can be edited the same way.
Open Pixel's widget picker
On a Pixel home screen, press and hold an empty area and choose Widgets. Find the app that supplies the widget. Tap its name to inspect the available sizes, then press and hold the widget and drag it onto a home-screen page. Lift your finger where there is enough room.
You can sometimes get to widgets by long-pressing an app icon too. That route depends on the app and launcher version. For the official sequence, see Pixel Phone Help: home-screen widgets. If a third-party launcher is your default Home app, follow that launcher's menus instead.
Resize a widget that supports resizing
Press and hold the placed widget, then release. A resizable widget shows a border with handles or dots. Drag those handles to expand or reduce it, then tap elsewhere to finish. Not every widget supports arbitrary sizes; the app chooses which size ranges and layouts it provides.
If the handles cannot move farther, try clearing nearby grid cells rather than repeatedly dragging. A widget's size also changes how much information fits inside it. A long calendar list may need a wider or taller space than a clock.
At a Glance is not an ordinary movable widget
Pixel's At a Glance area can display date, weather or contextual information near the top of the main home screen. Google says this built-in area cannot be moved like a normal widget on Pixel Launcher. Its content can be customized through the available settings, but you should not expect dragging to relocate it.
If you want a clock, calendar or custom photo elsewhere, open the widget picker and choose a separate widget instead. Compare the two on your actual screen before removing anything. The Pixel customization documentation explains At a Glance separately from widgets.
If a widget will not appear
Check that the provider app is installed and actually offers a home-screen widget. Place a simple built-in clock first to see if the picker itself works. If the widget appears but its information remains empty, that is usually a different issue from placement.
Follow missing widget troubleshooting for picker problems, or blank widget troubleshooting when its frame appears but content does not. Avoid clearing launcher data as an early experiment because it may erase your home-screen arrangement.