If you have an Apple Music subscription then there is no need to load the music files for songs that are in your personal library to the iPhone...the Apple Music library is vast and Apple Music will stream/download Apple Music copies of the music files you own it finds on your Windows PC to your iPhone from Apple’s servers. If you have no Apple Music subscription it just syncs your personal music files over to the iPhone. For music you have that isn’t available in their Apple Music library it should allow you to copy that over with iTunes for Windows.
https://support.apple.com/guide/itunes/sync-itunes-content-with-your-devices-itns5ecc4d98/windows
iTunes will not remain an option. Apple is replacing that with the Apple Music app along with Apple Devices. It is nothing like iTunes by my research. I’m open to being wrong. The newest release of iTunes doesn’t even display your connected device folders, music movies podcast and such. It’s not there. With the older version of iTunes, that is all displayed but you cannot do a simple copy/paste from the PC side of that to the phone side music folder. I bookmarked your link and will tackle my sons iPhone and attempt what they describe.