What's the difference between"quick start" and "iTunes/Finder" when transfer data?

Dear Apple Support,

I am transfer my data from my old phone to my new phone, but when I use "quick start" function, I cannot finish because the system always told me fail.

So I just try to use the Itunes/Finder to transfer the data, and it is success now.

My questions is: What's the difference between this two way ("quick start" or "Itunes/Finder")? Because My 200G data when transfer to new phone only 100G, I guess maybe the reason is that I need to redownload some apps.

Just want to confirm, thanks for your support!