is it a mix of all the courses in one year, and then you choose what you want to do for the remaining?
You do all the common courses that all engineers have to do in first year (so MATH11X1, MATH12X1, PHYS11X1, a computing course of which there are a few to choose but you only pick one, ENGG1000).
Then you have space to do an additional 3 courses, of which you will need to decide that will give you credit to your choices (like for example if you wanted to do anything in the school of mech or school of elec, then doing ELEC111X would be a good choice as it is a compulsory course for mech and elec students).
It is not a mix of courses. Its the common courses that all engineers must do (there's the five I named above), then 3 of your choice which can cover a range of disciplines as recommended. There are more specialised courses like PHYS12X1 which I have to do, but see that would only give you credit if you were doing a B Sci or anything in school of elec. So it would be better off to pick an elective that covers a wider range of disciplines as 6 UOC (like how I mentioned ELEC111X gives 6 UOC for all school of mech or school of elec programs).