If the files are from iTunes you'll need to know where on the hard drive they are (iTunes would copy to an iPod without you needing to know this, but it wouldn't to any non-Apple device). Start from "iTunes Music" or something like that and look through the subdirectories until you find them. Then you can manually copy them to the phone.
You can use Android File Transfer, but in my experience it doesn't play well with Smart Switch (i.e. on my laptop installing Smart Switch broke AFT). But you should be able to use Smart Switch to copy files too.
If the files were bought from iTunes they'll be aac rather than mp3, but that's not a problem: it's a standard file format (not an Apple-specific one) and any music player should be able to handle them.
There's typically a directory or folder named "iTunes Music" as Hadron says, but be aware Apple may have some weird filename convention that is iTunes proprietary, thus you might not see the song title as the filename the way you would expect with normal mp3 files on regular computing devices.
Once found, connect your S10+ by USB cable to the Mac and just drag 'n drop them to the phone's file folders. I usually create a top level folder called Music or Audio to keep all my music files on my phones for easy access.
As to a music playing app, I do use
VLC but in recent years I started using
Pulsar which provides a nice presentation interface that sorts songs by album | artist | genre and supports playlists.
