Is there a correct way to Sync?

[gerry_banks]

So sorry if this is a dumb question or has been asked already. I am a newby to MM5. Here are the particulars:
I have a very large playlist (18000 tunes) that I want to sync with my iPhone 11. I am running a MS Surface 7 Pro. So I send the playlist to the phone and it starts to sync. I cant do the whole playlist in one sitting. So I stop it after say copying 4000 tunes. I come back later and start the sync again, thinking it will just pick up from where it left off. But MM says it is copying right from the beginning again. There are no duplicates on the phone though.
So my questions are, am I doing something wrong here (setting)? Is there a better way to sync? Etc
Any constructive feedback would be most appreciated as I truly and new to MM.
Thank you all

[Lowlander]

How exactly do you Sync, using Auto-Sync it should only Sync those files that haven't been Synced yet.

[gerry_banks]

I was not using the "auto-sync" feature. I was doing the "Send to ..." feature. I will give your suggestion a go. Thank you

[Lowlander]

Send To will copy the files again.