WhatsApp lets you upload your chat history to either icloud or google's cloud offerings. Those backups are accessible to Apple/Google. Many people have enabled it, so if you are in any decently sized group chat you can basically assume it's available to Apple/Google, the people who hack them, and whomever they have data sharing contracts with.

Personally I use Telegram because they don't mind non-official clients like the FOSS builds. Whatsapp on the other hand is known to ban non-official clients. API access isn't available either unless you are a big corporate user, at which point you need a facebook ads account and cooperate with some API reseller... no joke lol.