Using immersion material can be pretty tough away from your PC. Fortunately there is an easy way to look up unknown words by using some of the built in features of the iPhone. This works for any language, not just Japanese.
I created an iOS Shortcut script that takes a screenshot and translate what’s on screen. This is activated just by tapping the back of the phone twice.
Demo: https://imgur.com/yuQXbg4
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Here is the shortcut script: https://imgur.com/xWyO0Ue
Note: You should specify Japanese instead of Detect Language so it ignores English. You can also add “Vibrate Device” as the first action so you get a confirmation that the Back Tap was registered.
Once created, you can go to Settings > Accessibility > Touch > Back Tap. You can then set the shortcut to run on a double or triple tap. You can also run the shortcut by saying “Siri, [Name of Shortcut]”.
When used, a text box with the translated info is shown on screen. If this is enough for you, then you can hit Cancel and the script exits. If you hit Done, it will open the screenshot in quick look, which allows you to translate in place. It also allows you to highlight specific words to open in the Apple Translate app to play audio. I like to favorite specific words so I can open the Translate app when I’m done to add them to Anki.
Sometimes, the screenshot from the script doesn’t use quick look properly and you won’t be able to translate in place [you can still highlight words!]. If so, you can just take a normal screenshot: https://imgur.com/lSFf3Sk. Sent a bug report to Apple so hopefully it gets fixed.