When do you use what? 信上/信里 and 写上/写下

When learning a new language, or when reading/writing something, are you often stuck at trying to understand what something means, or often wonder whether what you've wrote is correct, and hope someone can answer your doubts and questions?

You might want to try out HiNative, a global Q&A platform where you can ask people any questions on language, culture, and/or travel related. It also has an app, but because I have problems logging in via Lang-8, I uninstalled it. (Disclaimer: while I admit it's helpful, do note that not all answers are ... correct / what you expect.)

In HiNative, I came across this question from someone: "How do you say this in Simplified Chinese (China)? write your name on the letter"

Various people replied, including me, and the answers were roughly the same:
1. 在信上写上你的名字。
2. 在信里写下你的名字
3. 在信上写下你的名字
4. 在信里写上你的名字
5. 在信上写你的名字

As you can see from the above, the two major differences were "信上" vs. "信里" and "写上" vs. "写下", with the last omitting "上/下" altogether.

I can't find anything on "信上" vs. "信里", but searching for "写上" vs. "写下" returned a result from HiNative:

“写上”的语气强硬一点，一般是必须要写的东西，比如
老师对学生说：“作业本请写上名字。”
“写下”可以理解为语气稍微委婉一点，比如出去玩之后，老师对学生说：“写下你们的感受。”
不过虽然“写下”的语气委婉一点，也并不代表就可以随便拒绝。

As the person who wrote the above mentioned, it's interesting how we just use the words and never really thought about their differences. And when someone asks about it, we either don't know the difference, or find it so hard to explain it clearly.