There was no Apple Installer in the earlier days of the Mac. Only with the advent of System 6 did the Apple Installer begin to take its shape and form.
Apple Installer (System 6)
The Apple Installer was a dialogue box under earlier versions of the system. It did not permit the user to switch back to the Finder once it was launched. Multiple selection of individual items in a Custom Install (as opposed to an Easy Install had to be done -- trickily -- by pressing the Shift key.
Apple Installer (System 7.5)
The Apple Installer evolved under System 7.5 -- greatly. Instead of being a dialogue box, it became a separate window, with title bars, resizable windows, and even info buttons and the hierarchical triangular list view copied from the Finder. It also offered the option of a clean install.
Apple Installer (Mac OS 7.6)
The Apple Installer continued to evolve under Mac OS 7.6. It now is an "umbrella" installer, including routine that had to be otherwise independently executed, such as disk checks.
Apple Installer (Mac OS X)
The Apple Installer appears as a centralised application programme under Mac OS X. Whereas the Classic Mac OS used to include the Installer application along with the to-be-installed software bundle, Mac OS X includes only the software bundle as a package. The Installer would be installed on every Mac OS X computer, and it would run from the hard drive.