Visual novels on iOS
Non-jailbreak: App Store
There are a couple of native iOS "visual novels" but these are usually spin offs/gacha cash grab trash. There are a few iOS ports of Windows visual novels which you can buy at the Japanese App Store. You can switch to the Japanese App Store by creating an Apple ID with the "Japan" region. App Store accounts are separate to iCloud accounts so you don't have to switch your Apple ID completely, just in the App Store only.
Non-jailbreak: Using PPSSPP Emulator
While there are in fact native iOS "visual novels", these are usually gacha trash, and nobody wants to play them. That's why I feel emulating PSP visual novels is a better idea, just because the games on a PSP console are just going to be so much higher quality than games made for phones.
It is a little more tricky to get PPSSPP working on iOS without a jailbreak, but it is definitely possible.
You will need:
AltStore - check the FAQ on the website for instructions
PPSSPP IPA
Non-jailbreak: Kirikiroid2
Any games that use the Kirikiri2 engine (e.g. has .xp3 files) can be loaded and played natively without any virtualization on iOS.
You will need:
AltStore - check the FAQ on the website for instructions
Kirikiroid2 IPA
I no longer use an iPhone. The instructions are the same as Android anyway, only difference is you have to use AltStore to install it, and iTunes to transfer files.
Jailbreak: Using PPSSPP Emulator
PPSSPP can be installed via Cydia package by adding the repository: https://cydia.ppsspp.org/.
Tap here to launch Cydia and add the repository