xx learned to his astonishment that they were from North Korea.这句话里“learn to"是何意?

察觉

to his astonishment是插入语。
xx learned 【to his astonishment】 that they were from North Korea.
译文：让他惊奇的是某某人得知他们来自北朝鲜。

learn是发现的意思，to是和his astonishment 一起的，意为令他吃惊的是。句意为某某发现令他吃惊的是他们来自朝鲜

你理解错了，不是learn to,应该是learn that，to his astonishment 才是一个词组，整个句子可以翻译为：他惊讶的知道了他们来自朝鲜

to his astonishment “使他惊讶的是.”如果像这样放在句子中间,前后应该有逗号与句子分开,它一般置于句首.即：
To his astonishment,xx learned that they were from North Korea.
使XX惊讶的是,他得知他们是北朝鲜的.