How secure is Firestore / Google Cloud server-side encryption? We use Firestore. On their Google help page they say it is AES encrypted.

https://cloud.google.com/firestore/docs/server-side-encryption

But this means in the moment a user is logging in, the data is decrypted on the server not the client? So if there is a corrupt Google admin guy, could he wait till I login and then get the decrypted data?
Server-side encryption means that the servers, owned by Google and running in a datacenter rented or owned by them, has access to the encryption keys when you log in. Compare this with client-side encryption, where only the client has access to the key and the server reads and writes opaque ciphertext. So yes, a corrupt sysadmin or someone with sufficient access to the server your data is stored on would potentially be able to obtain the encryption key and plaintext data.