They can't read the messages.
I'd avoid the notion of "sharing with Facebook" as WhatsApp is Facebook. Instead I'd focus on what's used for advertisement targeting.
Here, metadata (data about the message, not it's content) that they are able to and already are collecting (e.g. who you're sending messages to and when) have been used for various things (e.g. preventing grooming by pedophiles), but not for advertising.
The new policy is controversial, because it technically allows this metadata to be used for advertising as well. They claim this only metadata that is going to be used this way your communication with business using WhatsApp - if you message a bike repair shop, expect to ads see ads for new bikes on FB or mobile apps using Facebook's ads services. But Facebook still wouldn't be able to read the messages, they just know you've now showed interest in bike repair - same as if you went to/liked the shop's Facebook page.
Additionally, businesses on WhatsApp will be able to store the message logs (not sure what this includes) on FB's servers for better access to them - also securely and without FB reading them.
Here's a twitter thread of Head of WhatsApp Will Cathcart explaining some of it: https://threadreaderapp.com/thread/1347660768225841152.html
The Verge article with explanation and sources: https://www.theverge.com/2021/1/12/22226792/whatsapp-privacy-policy-response-signal-telegram-controversy-clarification