To insert a special symbol, create a text replacement shortcut
Copy the symbol from anywhere on your iOS device. Open the Settings on your device. Tap General, then Keyboard, then Text Replacement. Tap the "+" at the top-right to create a new item. Tap and hold next to Phrase, then select Paste from the menu. Tap next to "Shortcut" and enter a word to be replaced with the symbol. Choose a word that isn't likely to be used for anything else. Tap "Save" in the top-right corner to complete. Your shortcut will automatically expand into the symbol when you type.