For an application appropriate to an 8051, you can generally get perfectly adequate "multitasking" from a very simple scheme such as:

void main( void )
{
   do_initialisation_stuff();

   for( ;; )
   {
      task1();
      task2();
      task3();
      task4();
      task5();
   }
}

In other words, each time round your main loop makes one call to each "Task" in turn.

Each "Task" is implemented as a function; each time the "Task" is called, it does a bit of work, and then returns.

Obviously, you have to take care to design your "Tasks" such that they can never hog the processor fo r too long.

Essentially, this is the way MS Windows 3.xx worked.

Hardware device drivers can be implemented as ISRs, communicating with the "Tasks" via appropriate buffers and/or flags.

It's not exactly rocket science!