The Mac could originally only run one major application at a time - although you did have 'desk accessories'. Andy Hertzfeld relatively soon wrote 'Switcher', a hack to allow multiple applications, which was released some time in early 1985. That wasn't virtual memory, it was real physical memory. The Mac wouldn't officially get virtual memory until System 7.

A more official solution wouldn't come until MultiFinder in 1987, and that was originally limited to one foreground and one background application. Wikipedia says:

"When an application is activated, all of its windows are brought forward as a single layer. This approach is necessary for backward compatibility with many of the windowing data structures that were already documented."

"With the release of System 7 [in 1991], the MultiFinder extension was integrated with the operating system, and it remains so in Mac OS 8 and Mac OS 9. However, the integration into the OS does nothing to fix MultiFinder's inherent idiosyncrasies and disadvantages."

By the time that OS X came out in 2001, the menu bar being at the top of the screen, and switching with the current app, was fully entrenched. After all, OS X initially had no native apps, still emulating the old OS 9 environment, and Carbon (essentially recompiling a "classic" Mac OS 9 app with relatively minor changes) would be the dominant API for some time too. Indeed Carbon wasn't removed until macOS 10.15 Catalina in late 2019.