You install a keyboard app, open Messages, and the same grey keyboard comes up. Nothing has changed. This is the point where the app looks broken.
It is not. On iPhone, a keyboard app installs an extension that sits switched off until you enable it in Settings, and even then you have to switch to it while typing. Two steps, neither advertised.
Here is the whole process, what the Full Access toggle actually does, and the places where iOS will refuse to show your keyboard no matter what you do.
Turning the keyboard on
Install the app from the App Store and open it once. Many keyboard apps set up their themes on first launch, and opening the app also registers the extension with the system.
Then go to Settings > General > Keyboard > Keyboards > Add New Keyboard. Apple's languages are listed at the top, with a Third-Party Keyboards section below them. Tap the one you installed and it joins your list. Themed keyboards such as Kawaii Keyboard & Wallpapers follow exactly this path, with no separate installer and nothing to sideload.
If the app is installed but does not appear there, open it once more and check again.
Switching to it while you type
Open anything with a text field and tap into it. At the bottom-left of the keyboard is a globe key. Tapping it moves to the next keyboard in your list. Pressing and holding it brings up a menu of every keyboard you have enabled, so you can jump straight to the one you want.
If you have only ever used one language and no third-party keyboards, there is a smiley face in that spot instead. The globe appears once you have more than one keyboard to move between. Whichever you pick tends to stay until you switch again, because iOS reopens the last keyboard you used.
What Full Access actually does
Tap your keyboard's name in the Keyboards list and you get a single switch called Allow Full Access, with a warning next to it.
Without Full Access, the keyboard extension runs in a tight sandbox. It cannot reach the network, and it cannot read the storage it shares with its own app. In practice that means themes, packs or GIFs you set up inside the app may never reach the keyboard, anything that searches online will not work, and key haptics are unavailable, because iOS only allows a keyboard extension to trigger them with Full Access on.
With Full Access, the extension can reach the network and its container app. That is what makes those features work, and it is also the honest cost: a keyboard extension can see what you type in the fields where it is used, and Full Access is what makes it technically possible for that to leave the device. iOS cannot tell you what a particular developer does with it.
What you can do is read the app's privacy policy and the privacy details on its App Store page, favour apps that state plainly that they do not collect what you type, and remember the switch works both ways. You can turn Full Access off at any point and carry on typing with fewer features.
Where iOS will not let a third-party keyboard in
Password and other secure text fields switch to Apple's keyboard automatically. You can watch it happen as you tap into the field. This is built into iOS, no app can override it, and it applies whether or not Full Access is on.
The device passcode screen is always Apple's.
Payment sheets, including card entry for Apple Pay, use the system keyboard.
Individual apps can also opt out entirely. Banking and some workplace apps do this deliberately, and when they do, your keyboard will not appear anywhere inside them.
Dictation belongs to the system keyboard, so the microphone key may be missing, or may hand you back to Apple's keyboard while you speak.
Put your keyboard directly after the one you type in most
In Settings > General > Keyboard > Keyboards, tap Edit and drag the list into the order you want. The globe key cycles through your keyboards in exactly that order, so if your themed keyboard sits immediately after your main language, switching is a single tap, and switching back is one more. It sounds trivial until you count how often you do it.
The small frictions worth knowing about
Autocorrect and predictive text come from the keyboard you are using, not from Apple, so their quality is a property of the app. Words your iPhone learned while you used Apple's keyboard do not carry across.
The first time you type after restarting the phone, a third-party keyboard can take a moment to appear, because the extension has to launch first.
Features you may think of as standard, such as one-handed layouts or gesture typing, are up to each keyboard to implement. If one is missing, that is the app rather than a setting you overlooked.
To remove a keyboard, go back to Settings > General > Keyboard > Keyboards, tap Edit and delete it. Deleting the app removes its keyboard too.
If it still does not show up
Work through it in order. Open the keyboard app itself. Check the keyboard is listed under Settings > General > Keyboard > Keyboards. Try typing in Notes, which does not opt out, so you can tell whether the problem is the keyboard or the app you tested in. Force-quit that app and reopen it. Then restart the phone.
If the keyboard works in Notes but never appears in one particular app, that app has opted out and there is nothing on your side to fix.
Quick answers
Do I have to turn Full Access on?
No. The keyboard types without it. What you lose are the features that need the network or the app's shared storage, which usually means downloaded themes, search and key haptics. Turn it on if you want those, leave it off if you would rather not, and change your mind later.
Why can I not use my keyboard for passwords?
iOS forces secure text fields back to Apple's own keyboard so no third-party extension sits in the path of a password. It applies to every third-party keyboard on every iPhone, and is not a bug or a permission you can grant.
My keyboard disappeared after an iOS update.
If a keyboard disappears from the list, re-enable it in Settings. Re-add it under Settings > General > Keyboard > Keyboards > Add New Keyboard. If the app needs an update of its own for the new iOS version, install that first, then re-add.
Where to get it
If you would rather practise than read, Kawaii Keyboard & Wallpapers installs exactly the way described above, and brings pastel themes plus matching wallpapers and GIFs once it is running.
Add it, reorder the list, and try it in Notes before you judge it in a real conversation.