Q: Iphone 6s 16g

Hello all

I just jumped into this century last Thursday by getting rid of my flip phone for a Iphone 6s with 16gb.  After it was set up I had around 11.2 gb left and that included around 25 purchased Itunes music. I additionally loaded around 30 or so contacts (just names and phone numbers) and the operating manual to the Ibook app., memory left was around 10.4 gb.  Yesterday I now see I have 10.0gb, this doesn't make sense to me can the contacts and manual be almost a gb?  I delete messages after reading and phone calls, I also turn off bluetooth and data when not needed.  I went thru my settings, is there something I didn't turn off that maybe using up memory?  Also this phone came with apps I will not use and wish I could delete them and hope upgrades don't automatically download to them.  Any thoughts would be appreciated.

Regards

Steve