Most of the mosques in China and India were not originally mosques they were Hindu temples that were converted over. Even the Taj mahal was not originally mosque.
Islam initially reached China through merchants which is not a hidden fact I think you are talking about a later time in history.
It is prohibited in Islam to change someone’s place of worship into a mosque by force.
If it happened it is either due to human shortcoming or the people willingly converted it to something else.
For example , when Omar (the best friend of prophet Mohammed ) conquered Jerusalem from the Byzantines he refused to pray inside the church in fear that the Muslims will take that as a sign to change the church into a mosque This is not a hidden story you can look it up.
By the way I can give a non ending list of examples of churches who were initially not churches .
