What must the application do when it is finished running?
Pass control back to the operating system.
With Microsoft's DOS, programs were passed control, ran until completion, then passed control back to DOS. So only one application ran at a time. Worse, an application that messed up might never return control to DOS and the whole system would freeze.
Modern computer systems include features that can interrupt the control flow of an executing program. After the flow is interrupted, the operating system can give control to another application. If this is done many times a second the computer system appears to be simultaneously executing several applications. This trick is called multitasking. It has been used from about 1960 for mainframes, from 1978 for many microcomputers, and from about 1995 for Windows PCs.
The MIPS chip has very good support for multitasking. But this is an advanced topic not included in our basic programming model.
Do human brains do multitasking?