A big point of a NAS in my mind is to run some sort of redundancy, which means you will want to setup a RAID on the drives in the NAS, and that in turn means that my recommendation wouldn’t be to chuck existing drives into the NAS solution but to setup the NAS drives and then copy your data to it.
Dedicated NAS hardware storage is usually accessed over SMB, NFS or SFTP and most software has support for one of those protocols.
Some services can have hiccups when running against networked storage, f.e. Jellyfin might lose library metadata if the Jellyfin service’s library scan is started and the networked storage is unavailable.





Depends on the operating system of the NAS, but generally the NAS will want to format the drive. Even if you can somehow get it running without a disk format you’re generally in an unsupported configuration.