Will enabling FileVault convert my HFS+ disk to APFS?
FileVault and APFS are two very different things; enabling one doesn’t enable (or necessitate) enabling the other.
FileVault is encryption - it scrambles, to put it simply - the contents of your drive with a known key.  Specifically, it uses a 128bit AES encryption with a 256bit key.  See Use FileVault to encrypt the startup Disk for additional details.  You can encrypt JHFS+ formatted volumes as well as APFS volumes.
APFS (Apple File System) is simply Apple’s next generation file system.  As far as encryption goes, it supports it natively unlike JHFS+.