It seems that Korea is much more Christian in its history than the rest of east Asia. Christianity didn’t have a lot of success in China before the CCCP went state atheist and Japan outlawed Christianity in favor of Buddhism and Shinto which it seems they continue to practice outside of seemingly western/Christian style marriages. In the US I’ve seen specifically Korean Christian churches but never the same for Chinese or Japanese Christians.
This question was asked a couple times before.
u/ARPK describes the rise of Protestantism within late-Joseon, under Japanese occupation, and under liberation. Protestantism was introduced minorly by the non-elite class and exploded under Japanese occupation as a co-opted expression of Korean national identity. American or western involvement in the spread of Protestantism was negligble.
u/iveymikey gives a detailed analysis of the seeds of Catholicism within Late-Joseon. It was adopted by the namin faction of the Yangban elites but was suppressed by the Joseon court for various political reasons. He also explains the rise in christianity in post-war modern Korea - which is related to the links between Korean Christian groups and labour rights movements. American or western involvement in the spread of Protestantism was negligble.
u/dsk_oz provides a really in-depth view of Christianity within Korea in a two-part comment series (and several more replies below). He notes the differences between the seeds of Catholicism and Protestantism in late-Joseon. Due to these differences, these are seen as separate religions within Korea, rather than sects of the same. Catholicism was introduced through text by the literati class, but persecuted by the royal court until 1876 . Despite this, Korea had the 4th largest number of catholic saints and is the "only country where catholicism emerged without foreign intervention". Protestantism was mostly introduced after the opening of Joseon's isolationist policies in 1876. The royal court's primary interest was utilizing the western-style scientific skills brought by Protestant missionaries. They established many of the first modern educational institutions (i.e. universities) in Joseon, but were discouraged from proselytizing. Religious texts were transcribed into Hangul and spread primarily through text amongst the laymen, mirroring the spread of Catholicism amongst the Yangban class. He skips over the history during Japanese occupation and post-independence Korea, and describes the current-day dynamic of these religions.