To use SUMIF from another sheet, you simply reference the sheet name in the range and the sum range.
An example of usage would be:
=SUMIF(Sheet2!B3:B11,"Apple",Sheet2!C3:C11)
As you can see in the above example, the formula will sum up all the values in Sheet2 if they are “Apple“.
Sheet2!B3:B11 refers to the data range you are checking against
Apple refers to the pattern to apply against the range.
Sheet2!C3:C11 refers to the range to be summed, if it is different from the 1st data range.