What's wrong with applying for a patent abroad, one country or all countries?

You can apply separately in different countries according to the Paris Convention.

You can also file an international application according to the PCT Convention and then enter different countries.

It depends on how many countries you want to apply for. If more than five countries apply, it is more economical to use PCT convention. To apply abroad, you must entrust a patent agency, and you must pass the confidentiality review in China National Intellectual Property Administration.