Typing on my iPhone: how to change input method when typing from an external Bluetooth keyboard

I need to type in two languages on my iPhone but can only do so easily when typing from Apple's "Magic Keyboard". I am looking for potential ways to make typing on an iPhone easier for other Bluetooth keyboards that were not made by nor for Apple devices.

I should be running the latest iOS software.

The Trouble: For now, after I paired the iPhone with an Always-On Bluetooth keyboard at home, I am forever bound to such keyboard: it won't quit, as the keyboard is powered by a set of AA batteries and there is no switch on it to turn it off, and it won't let me change input method!

What the "Magic Keyboard" does when paired with iPhone

Thanks to the integration of Apple products, there are two ways:

1. At the far right corner of the Magic Keyboard, there is a key that can show/hide the on-screen keyboard. One can use this interface to switch input method.
2. Moreover, the default "Mac-OS" keyboard shortcut still works: holding opt + space shall get me to loop through the input methods

Yet, I only have bought one pair of Magic Keyboard and have kept it at work. Over the years, I should have a good number of generic Bluetooth keyboards, 2 among which are fancy mechanical switch ones. Unfortunately, they are "generic Bluetooth keyboards" ==>

What generic Bluetooth keyboards do? And, can we avoid buying

Well, when typing, I did not find a way to summon the on-screen keyboard at all. This bans all graphical interface tweaks: there is simply no button to click on.

• Question 1: Can I devise finger swipes to change input methods? I have only two in total.

Then, Alt + space does not register on iPhone. This may have to do with how these generic Bluetooth keyboards are dealing with issuing keystrokes, and I wonder if there is a walk-around? Say:

• Question 2: Is it possible to define keyboard shortcuts for iPhone to loop through input methods?
• Question 3: Or, can I spare a F1 key if key-combination is not allowed, solely for iOS to change input method.

The standard way to switch layouts in iOS is always Control plus Space (not Alt/Opt). It should work on arbitrary keyboards with these two keys.

There’s no way to change this shortcut in iOS as far as I know.