There are several methods for doing it but they are all crippled in ways that make it impossible to actually use the system for anything but QA testing and development.
You can install any app you compiled with your own key and it lasts for 7 days before requiring it to be recompiled.
AltServer can do it but in my experience the Windows version of AltServer is extremely unreliable. AltStore never really auto-renewed properly for me until I put AltServer on an old MacBook Air. Furthermore, the fact that this process needs a second device to bootstrap provisioning at all[0] might be a non-starter for some. Take a week-long vacation? Well, now all your sideloaded apps stop working.
A far bigger limitation for me is the three-app provisioning limit. There isn't any way to work around it[1], and if you do want to do serious sideloading you almost certainly will need to upgrade to a paid developer account.
[0] Specifically iOS only allows app provisioning over USB or Wi-Fi, not locally. Locally installed software cannot actually communicate with the remote debugging daemon. You can work around this with network extensions, but you don't get to use them in dev-signed apps unless you have a paid dev account that's been approved by Apple to use them.
For the record, that isn't to make sideloading harder; that's because Facebook went and shipped a spyware VPN with their enterprise cert.
[1] Personal experience time: Even when jailbroken, and with AltDaemon and Immortal installed, AltStore still bumps up against the three-app limit.
You can install any app you compiled with your own key and it lasts for 7 days before requiring it to be recompiled.