A type of "machine translation" called Phrase-Based Statistical Machine Translation (PBSMT) was commonly used up until the first half of the 2010s. PBSMT generates a translation by combining individual word strings, but often resulted in issues with overall fluency because sentences are composed by cutting and pasting these word strings.
Mirai Translator uses Neural Machine Translation (NMT). Instead of working with individual word strings, NMT converts entire sentences into numeric data, and then generates a new translation from this numeric data. This can provide a more fluent translation result than PBSMT.
Features of neural machine translation
Refer to "Translation Tips" for more information.