You can download .ipa files of iOS apps using iTunes 12.6.4. This is a special branch from Apple that can be installed alongside newer versions of iTunes. https://support.apple.com/en-us/HT208079
The Mac version is not compatible with macOS 10.14 Mojave or above. You may want to use the Windows version inside of a VM.
Once you have the .ipa file, you can peek inside further by changing the file extension to .zip and opening it with Archive Utility. Then, right click the .app file and select "show package contents". This is useful if e.g. you want to extract an icon or other asset.
I haven't used this myself but you can also download the .ipa files with iMazing, which is compatible with current macOS releases. However, this requires putting your Apple ID password into a third-party program.