cannot fins the device management on my settings. I have an xr

Go to Settings/General. Scroll to the bottom. Look just below VPN. If your phone has a device management profile it will appear there. If there is nothing below VPN except regulatory you don’t have a device management profile.

If you are trying to install an app that is not in the app store that would be an incredibly stupid thing to do, unless it was an enterprise app provided by your employer. If it is an employer-provided app they should give you a link to install the required profile.

With over 2 million apps in the app store you should be able to find a safe app for just about any use. Apps that are “side-loaded” and that are not available in the app store have a high probability of containing viruses or spyware.