Adding numbers in a single cell without using formula bar

kaizo.... I think you have a fundamental misunderstanding about how Excel works.

First, features like Autosum are merely alternatives ("shortcuts"?) that Excel provides for "convenience".  Nothing requires that you use them.  I never do.

Second, the Formula Bar is simply a place where Excel shows the content of a cell.  In other words, we type into a cell, and it appears in the FB.

Sometimes, it is convenient to change the cell content by editing in the FB.  That is the way I operate. 

But if you choose the appropriate option ("allow editing directly in cells"), you can eschew the FB and type directly in the cell.  Select the cell, then press f2, then edit.

As for adding numbers in a single cell -- and performing any arithmetic -- that is the purpose of formulas.

It is not as simple as ``automatically sum numbers that you type into that specific cell``.

Instead, we have to write an expression, preceded by an equal sign ("=").  For example:

=123 + 456 + 678 + 91011

BTW, some people write =+123 +....  The first plus is totally unnecessary.

However, some people want to use only the numeric keypad for entering such formulas as well as numbers.  And apparently, the numeric keypad does not have an equal sign.  (I do not have an NK, and I would not use it, if I did.)

For their benefit, Excel allows us to enter formulas starting with plus ("+") or minus ("-") instead of an equal sign.