Why doesn’t Profiles & Device Management show up on my phone?

Probably because you don't have any profiles and don't have device management. These folders only appear in Settings/General if you have installed an Enterprise profile or if your device is under management from an employer. The folders will be in Settings/General near the bottom, just after VPN if they exist.

Profile and Device Management will show up on those iPhone where a profile has been installed by an organisation / enterprise / corporate to safeguard their data.

If it is an individual iPhone the if you install an which requires you to update a profile to run the app then you will have it.