I have this problem on two phones an iPhone 12Max Pro and an iPhone SE 2020, I also had it on my previous iPhone 7 Plus. I can be using my MacBook on the same Wi-Fi at the same time and only the phones have a problem. A quick switch off of my Wi-Fi on the phone and back on a connection is fine again. My daughter also experience the same problem on her IPhones at her home. Apple need to do something as it is obviously a problem with the way Apple check the quality of the Wi-Fi connection.

I have a similar problem on a Mac Mini 2014 i7 it works fine for a while the suddenly finder stops responding normally, if I try to open a new finder window nothing happens, if I click on the finder icon nothing happens. However if I double click a drive on the desktop a finder window opens, I still can't open a new one and now if I click the icon it moves to the open one. Also occassionally the HDMI loses sound I jest get clicks all other devices still work just HDMI sound goes. It's not always convenient to restart so at least I have a work around. Big Sur is definitely not as reliable as the previous version and often hangs for a while on the coloured ball of death, sometimes it recovers sometimes the only option is power off with the 4 sec hold on the power button.