ところ＋particle(助詞) precision

Question

Hy, I am currently working on with 新完成マスター N3 for my grammar part and I get stuck on a small section of the lesson related to ところ(~p.17)。

So far, I understand that, according to this lesson, ところ can be used to express something that just, will or be happening soon.

However, I do not get the point of the particles added after. I can understand why we would use ところで/ ところだ、but not why we would use ところを/ ところへ。

Here are the 2 sentences use as an example that I don't quite get the use f the particle:

1. ケーキができ上かったところへ子供たちが帰ってきた。
2. 試験中、隣の人の答えをみているところを先生に注意された。　

Again, what role do those particles play exactly and how do they influence the sentence.

Thank you for your time.