So make it 3mm thicker?
The iphone 13 is 7.65mm. The Unihertz Jelly is 16.5.
Apple hasn’t made a phone thicker than 10mm since the 3GS, and that was 12.3 (up from the original 2G’s 11.6 because of the rounded plastic back vs flat aluminum).
What do you mean by newer iPhones? They use stainless steel since 2017 in the models X, XS (Max), 11 Pro (Max), 12 Pro (Max) and 13 Pro (Max), so it's nothing new.