How secure is Firestore / Google Cloud server-side encryption?
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.