For many years now, the default behavior on macOS has been to completely hide scroll bars unless you are actively scrolling. This drives me crazy because you cannot tell what parts of your screen are scrollable unless you move your mouse around and try scrolling things, and because you cannot see how large a document is at a glance. Luckily, Apple has a setting to turn the scrollbars back on for good.
On macOS 13 Ventura or later, go to the Apple menu > System Settings > Appearance. Switch "Show scroll bars" to "Always".
On macOS 12 Monterey or earlier, go to the Apple menu > System Preferences > General. Switch "Shos scroll bars" to "Always".
Automatically based on mouse or trackpad will only show scroll bars if you are using a pointing device that doesn't support gestures (such as a third-party mouse or an older Apple mouse) or when actively scrolling. When scrolling will only show scroll bars when scrolling. Always will show your scroll bars all the time.