WeChat (Weixin in mainland China) is an app from the Chinese company Tencent that combines messaging, voice and video calls, group chats and a social feed (Moments) with Official Accounts for organisations, mobile payments through WeChat Pay, and Mini Programs, small apps that run inside the app. It is a consumer platform used as a smartphone app, and it also reaches businesses, for example through accounts for publishers.
A separate privacy policy applies to users outside mainland China. For users in the EEA, the UK and Switzerland, Tencent International Service Europe B.V. in Amsterdam is the data controller. According to that policy, servers are in Singapore and Malaysia, and in Hong Kong for users in Hong Kong and Macao. Transfers from the EEA to Singapore, Hong Kong, Malaysia and, limited to technical support and interoperability with Weixin, mainland China rely on standard contractual clauses. Wikipedia notes there is no end-to-end encryption. For a European organisation, this means data on the service is stored outside the EU and the operator is owned by a Chinese parent company.