The most reliable way these days is to use Cydia Impactor to install emulator IPAs straight from a PC to your iPhone. Though, you have to re-sign the apps when they expire after 7 days and are limited to only 3 apps at a time per Apple ID if you aren’t registered as a paid-for developer account.
The other way is to use a signing service, which doesn’t require a PC and you can install more than 3 apps at a time. There are some free public ones online with popular emulators like Provenance and PPSSPP but they are quick to get revoked lately, and it can take a varying amount of time for apps to get re-signed (sometimes it can take days or even weeks). So unless you use a private, paid-for signing service I personally wouldn’t use them.