Which BIP describes the structure and algorithm for creating a wallet file?
There is a list of BIPs at https://github.com/bitcoin/bips

I don't see anything that specifies a wallet format. 

To be honest I'd be surprised to see one. This is something that each implementation ought to determine for itself, Some wallets might use an embedded SQL DBMS, some a Berkely DB format, some a JSON store of some sort and so on. Similarly, the contents may depend on what unique features and facilities the wallet offers. Wallets might be multicurrency or even multiuser, they might have settings for GUI skins that need to be saved as part of a wallet backup, they might have IDs for associated exchange or other services.