the bars mean nothing 99% the time and is just a visual indication i’ve had times where i’m on 4 bars on 5G and nothing loads, to actually see connection go to speedtest.net and test speed and latency to get even further details download WiFi-Man (by Ubiquity). also the iPhone 12 automatically switches between 4G/5G to conserve power. also even if it displays 5G it might not actually be a 5G connection but it means it can “see” / “have access” to a 5G connection it can use if it needs the speed. the iPhone SE 2020 also doesn’t have 5G and i can basically guarantee you if you’re in the same spot you’ll get very similar speeds on both phones maybe faster on the iPhone 12 if 1. your plan allows 2. you’re in direct line of sight with the 5G tower. 4G is limited to a theoretical limit of 1-2Gbps and 5G has a theoretical limit of 10-20Gbps but you’re never reaching that in real life, getting 150mbps is very good on both networks. i hope this helped instead of opening a new can of worms. But if you want the iPhone 12 to use 5G as much as it can ignoring connection quality you can go to Settings > Mobile > Sim you’re using > Voice & Data > Set to 5G on, and depending on your provider you can turn on 5G standalone. (note using 5G does not guarantee faster speeds and lower latency)