I have a 49" ultrawide. Most websites simply center themselves in the window. Occasionally they will position on the left. And very occasionally, but most annoying of all, they will try to take the entire width of the monitor, or adjust their font and image sizing to fit the width, but not consider the height. Some websites will put UI controls on left and right, and content center, which sucks when you need to move the mouse four feet to click on something.

For the most part I specifically size the browser window to how I want it to be. On Windows there are programs that let me hotkey size and position of windows, which makes it easy to tile three browser windows across the desktop. Unfortunately nothing quite like that exists for macOS or Linux at this time that both understands the hotkey, how to apply it to a window, and will consistently apply the change on the 49" monitor but not on the laptop screen.