There are several ways to install apps that do not go through the app store.
Your company can have an enterprise certificate that allows in-house apps to be run on company devices without needing to go through the app store approval process or the app store. You are not supposed to use that enterprise certificate to publish apps to the general public, although companies have been caught doing so in the past.
If you have a paid developer account, you can test your own apps on devices owned by your company and your customers.
Anyone can get a free Apple Developer account that allows you to test apps on your own device.
From my understanding, don't you have to reinstall the apps every week since they expire and disappear from your phone? That's an awful experience for daily use.
There are several ways to install apps that do not go through the app store.
Your company can have an enterprise certificate that allows in-house apps to be run on company devices without needing to go through the app store approval process or the app store. You are not supposed to use that enterprise certificate to publish apps to the general public, although companies have been caught doing so in the past.
If you have a paid developer account, you can test your own apps on devices owned by your company and your customers.
Anyone can get a free Apple Developer account that allows you to test apps on your own device.