The first two of the problem set, A and B, are assumed to be the easiest.
They are in order of easiness estimated.
The order of the problem C and after is randomized irrespective of their difficulties.
Interactive problems are the same as other problems in a way that your program will read from standard input and print results to standard output. The difference is, the standard input and output are connected to a special program (judge program), with which you have to communicate back and forth. Unlike other problems where the input text is fixed for each test case, the input varies based on your previous outputs.
With interactive problems, it is crucial to make sure the output is actually sent from your program.
Please read “Output Buffering” section of the Judging Notes in ICPC World Finals for the details: https://docs.icpc.global/wp-content/uploads/2021/09/2020-21.JudgingNotes.Addendum.A4.pdf
We will provide an interactive problem in the rehearsal session held the day before the contest.