The default iOS only allows app installations from the App Store, but with the tricks outlined below, you can easily install IPA files from external sources onto your iPhone or iPad.

1. Installing IPA Files on iPhone and iPad using 3uTools
First, you need to install two tools on your computer: iTunes and 3uTools. You can find installation links here:
Download iTunes
Download 3uTools
Step 1: Connect your iPhone or iPad to your computer and open the 3uTools software. Then, click on the Apps (1) tab => Import & Install IPA (2).

Step 2: Click to select the IPA file (1) => Open (2).

Next, the installation process will commence, and you'll witness the status Installing displayed below the software.

Step 3: Once the installation is complete, you'll notice the app appearing on the home screen, but refrain from opening it immediately. First, navigate to Settings (1) => General (2).

Step 4: Tap on Device Management (1) => tap on the developer name (2).

Step 5: Select Trust '….' (1) => Trust (2). Once successfully trusted, you'll see the Verified status next to the app's name.

Step 6: Launch the application and experience it; success is when the app functions smoothly without crashing.
Note: Applications sideloaded with IPA files from external sources may have their certificates revoked by Apple after 7-14 days. Remember to back up your data before certificate revocation.

2. Sideloading IPA files using Sideloadly tool
Sideloadly is a tool for installing IPA files on iPhone, iPad without Jailbreak, serving as a replacement for the well-known Cydia Impactor tool. Firstly, install iTunes on your device before installing Sideloady. The download link for iTunes is provided above, while the link for Sideloadly can be found below:
sideloadly.rar
2.1. Installing Sideloadly
Step 1: Unzip the downloaded folder and open it.

Next, navigate to the corresponding folder based on your operating system, such as MacOS, Windows 32-bit, and Windows 64-bit.

Note: To install Sideloadly, your computer must be running Windows 7 or later and macOS version 10.12 (Sierra) or later.
Step 2: Open the installation file (1) and click Next (2). In the next step, click Next to install the application into the default folder.

Step 3: Click Install to begin the installation process.

Next, click Finish to complete the installation.

2.2. Installing IPA files on iPhone, iPad using Sideloadly
Step 1: Open Sideloadly. Then, select your device (1), enter your Apple ID (2), and click on the IPA icon (3) to open the folder containing the IPA file.
Note: The Apple ID used serves as the developer account, and you can create a secondary Apple ID to use as the developer account.

Step 2: Click on the IPA file you want to install (1) => Open (2).

Step 3: Click Start to initiate the installation and sideloading process onto your device.

Next, enter your Apple ID password (1) => OK (2).

Subsequently, the installation process will commence. You'll observe the Installing status below the Start button.

Successful installation will display a Done. notification as shown in the image below.

Step 4: You'll see the app icon on your iPhone's home screen, but refrain from opening it right away. Instead, open Settings (1) => General Settings (2).

Step 5: Choose Device Management (1) => select the developer's name (2).

Step 6: Tap Trust '…' (1) => Trust (2).

After successfully trusting the app, open it and experience it. Success is when the app functions smoothly without crashing.
Make sure to enjoy and back up your data while using this method because installations like these only last about 7-14 days, after which the app's certificate will be revoked by Apple and it will no longer be usable.

With the guidance provided in this article, you can experience paid games or modded games with features not available on the App Store. Best of luck to you all!