You know that there is a “TAB key” on the left edge of your computer keyboard. Don't know? Some computers, such as Mac, only have a symbol such as “→ |”, so you may have used it without knowing it. By the way, the TAB key has a special role and is used to insert several spaces (actually the control character code) or move between items.
The TAB key does not exist on the iPhone software keyboard. If you prepare an external keyboard (Bluetooth keyboard) or use an app with a tab insertion function, the problem can be avoided for the time being, but it cannot be like a computer keyboard.
If the purpose of using the TAB key is to insert a few spaces, there are several alternatives. One of them is copy and paste from a document containing TAB. You can open a document containing a TAB created on a computer on your iPhone, select the area of the TAB, copy it, and paste it into the target document. It may take time, but it is easy to use an app that can be synchronized with iCloud, such as “Memo”.
Another way is to use the voice input function. Move the cursor to the position where you want to insert the TAB, tap the microphone button at the bottom right of the software keyboard, and say "Tab key" toward the iPhone. If your tongue is bad, it will be frustrating when you enter “Kabuki”, but the advantage is that you can use it without any advance preparation.
Voice input will not function unless the voice input switch in “Settings” → “General” → “Keyboard” is turned on. Please note that unexpected words may be input depending on the voice input language setting.