How to switch between Khmer and English keyboards on iPhone
With both keyboards added, open a text field. On the Apple keyboard, touch and hold the Emoji or globe key, then choose iBoard. To go back, tap the globe on iBoard when it is shown and check which keyboard appears. If it opens another Apple keyboard, use its chooser to select English.

Start with both keyboards in a draft
Open a blank note where both keyboards are available. Use a short mixed-language example such as a book note that includes the Khmer word សៀវភៅ and the English label Shelf B. Start from English so you can check the Khmer layout when it appears.
If either keyboard is missing from the chooser, check the enabled list under Settings, General, Keyboard and Keyboards. Return to the note after both are available.
Choose iBoard from the Apple keyboard
With the insertion point in the note, touch and hold the Emoji or globe key on the Apple keyboard. Choose iBoard from the list. Check that the Khmer keys are visible and type សៀវភៅ.
Keep the caret in the same note when you switch. Add a space or start a new line where you want the English label to go.
Return to English from iBoard
When iBoard shows a globe, tap it to advance to the next keyboard. If iBoard shows its emoji control instead, touch and hold that control and choose its next-keyboard option. Check the layout that appears before entering the label.
If another Apple keyboard appears first, use its chooser to select English. Type Shelf B beside the Khmer word. If you need more Khmer text, open the chooser again from the Apple keyboard and choose iBoard.
Check the result in the same note
Read the Khmer word and English label back in the note. Switching selects the keyboard for the text you enter next. The iPhone’s display language is a separate setting, so the draft exercise can be done in the same app with the same interface language.
Questions
Why did another keyboard appear before English?
The next-keyboard control advances through the enabled keyboards. On an Apple keyboard, hold the Emoji or globe key to choose English directly.
Can I switch back to Khmer in the same note?
Yes. Leave the caret in the note and choose iBoard again from the Apple keyboard chooser.