Will any unlocked phones work on WiFi after unlocked

I have an IPhone 7 Plus that I am upgrading the line to an iPhone 11 Pro Max. It is in brand new condition and I would rather keep it than get rid of it because I did pay quite a lot for it. If I unlock it can I still use it with WiFi and will all my pics stored on my phone be accessible when not hooked up to att service? I don’t trust putting my info in clouds. I know I used to be able to keep any info and access it on my phones without service but things have changed so much.