computers are intricate to understand at the superficial level, but at its core all you will see are logic. Logic makes everything. The invention of transistors allowed us to create logic gates and those logic gates allow computers to add. couple of XOR gates and AND gates can create a full adder. but how do computers subtract? they use something called 1’s complement because computer developers thought “screw this , we are not going to build another circuit for subtraction, rather we will use same circuit board that we use for addition”. They achieved their word by using something called complement method.
so whats complement method?? Here is simple example:
Subtract 233 from 2342.
its pretty straightforward for us , the answer is 2342233=2109 but for computer
 Step 1 – specify subtrahend and minuend2342 (minuend)
233 (subtrahend)
step 2 if there are not equal number of digits make equal digits on both minuend and subtrahend
 2342 (minuend)
0233 (subtrahend)  step 3 find 9’s complement of subtrahend
9's complement = 99990233 = 9766
 step 4add 9’s complement of subtrahend and minuend
2342(minuend) +9766(subtrahend) 12108
if there are more digits on sum than subtrahend or minuend, in our case 12108. here minuend and subtrahend are 4 digit numbers whereas our sum is 5 digit so its left most digit is carry here in 12108 , 1 is carry so will write it as 1/2108
 if there is carry digit then :
 add the carry digit
(1)2108 //here (1) is carry digit so 2108 +1 2109 //answer
 add the carry digit
NOTE:
 if there is no carry digit then:
 again find its the complement of its sum and it will be the answer but in negative
Here is another example:
Subtract 12345 from 9898.
its pretty straightforward for us , the answer is 2342233=2109 but for computer
 Step 1 – specify subtrahend and minuend
9898 (minuend)
12345 (subtrahend)  step 2 if there are not equal number of digits make equal digits on both minuend and subtrahend
09898 (minuend)
12345(subtrahend)  step 3 find 9’s complement of subtrahend
9's complement = 9999912345 = 87654

 step 4add 9’s complement of subtrahend and minuend
09898(minuend) +87654(subtrahend) 97552
 step 4add 9’s complement of subtrahend and minuend
 since there is no carry digit so:
 again find its the complement of its sum and it will be the answer but in negative
9's complement of 97552 = 9999997552 = 02447
so the answer is 02447 due to mentioned reason above.
 again find its the complement of its sum and it will be the answer but in negative
so this is how computer takes your data and subtract it , but computer understand only binary language aka the language of 0 and 1. On next post i will be talking about 1’s complement method