What is the units digit of 3^1001 * 7^1002

Please explain the method for answering this problem. Thank you!

3^0 = 1
3^1 = 3
3^2 = 9
3^3 = 27
3^4 = 81

The units digit follow this pattern 1, 3, 9, 7, 1, 3, 9, 7, ...

1001 % 4 = 1; 3^1 = 3

so 3^1001 has a units digit of 3

7 has the same pattern, but in reverse

7^0 = 1
7^1 = 7
7^2 = 49
7^3 = 343
7^4 = 2401

1002 % 4 = 2; 7^2 = 49

7^1002 has a units digit of 9

so if 9 * 3 = 27, the units digit of the product is 7