Use Mission Control: swipe up with three or four fingers, or press Control + Up Arrow
Select the app window you want to use
Drag the window to the left or right side of the screen until you see the split preview
Release the window to snap it into place
Click/select the second app window to fill the other side
Use Stage Manager (macOS Ventura or later): click the Stage Manager icon in Control Center (or open it from Control Center)
Select the apps you want to use in Stage Manager
Drag one window into the split area, or use the split view option from the window controls when available
Use full-screen split (if supported by your macOS/app): enter full screen for one app (hold Option and click the green button if needed), then choose the split option for the second app
Exit split view: drag windows back out of the split, or use Mission Control to select another layout