When the time is fixed, changing the tense is possible but not necessary in reported speech. It changes the meaning slightly: “I worked on Christmas” (There could or could not be a present effect of the past event) I said that I worked on Christmas. (factual description of the timeline. Doesn’t say whether there was an effect on me when I said it) I said that I had worked on Christmas. (Clarifying that at the time I said it, I felt that there was an effect on me from having worked on Christmas.) ‘when he arrived’ fixes the time in the past, like ‘on Christmas’. “I was waiting when he arrived.” I said that I was waiting when he arrived. (Time line is correct) I said that I had been waiting when he arrived. (Conveying that there was an effect on me at the time of his arrival from having been waiting.)

Nope. no need to put the past simple into the past perfect here. "Simple Past and Past Progressive do not normally change in sentences with WHEN / IF" is an exception to the rule where you push the sentence back in time. You could actually say "I said that I was waiting for he bus when he arrived".

My version is I said that I had been waiting for the bus when he arrived.

I'm not an expert, but I think it's like this . . . . Saying "I said that I HAD been waiting . . . . etc." moves the sentence farther into the past. So you don't want to move 'he had arrived' even farther into the past, because the time relationship between you waiting for the bus and his arrival did not change. Another example: I was eating when a dog barked. changes to: I said that I had been eating when a dog barked. The overall sentence has changed, but the time relationship between EATING and the DOG BARKING has not changed. So, 'when a dog barked' does not change. I've never thought about this before. Hopefully this makes sense.

Hmm im newbie i need learn anything language sorry my english bad