IMPORTANT: F21 Pro smartphone comes out of the box with the firmware version 1.0.8 or 1.1.0 at the moment. In order to keep the ability to choose other languages than English and Chinese as the main for applications, to preserve the opportunity to install third-party APKs, as well as potentially to get root access, Magisk, and Google services in the near future - it is strongly NOT RECOMMENDED to update the phone over the air to the firmware v 1.1.7 (or older) until further instructions from Chinese masters of kung fu-rooting, as there is no way to rollback the FW yet.
Also, go and enable developer mode in the menu and uncheck Automatic system updates in Developer Options to avoid that.

QIN F21 Pro is a full-fledged 2.8-inch Android 11 smartphone in the format of a classic push-button phone, while it has an IPS display, multi-touch screen, GPS, BT5.0, IR blaster for technics control, dual-band Wi-Fi, two cameras, and all this based on the MTK A22 chipset . Obviously, the device is the best in its class - having the responsive interface, good battery life, being comfy in the hand.
F21 Pro is positioned by the manufacturer as the phone for students, hence it has closed (and keep closing in the new versions of the firmwares) routines to install entertainment and third-party software, so there is no legal solution to install apk locally or through adb, just using the native Chinese market only having educational and application programs.

Instructions of how to install your APK on the smartphone having the firmware versions 1.0.8-1.1.3 (closed on FW 1.1.7 and later) by APK substitution method.
At the moment this is the only working way to install the third party software on the F21 Pro, adb install is disabled by the manufacturer.

1. Download the APK we need, preferably Lite or Mod versions with Google Services unbinding. Put them in a folder.
2. Connect the phone to PC and set the device to removable storage mode (to avoid choosing this mode manually in future, activate Developer options -> Default USB configuration -> File Transfer ON).
3. Keeping open the folder with our APK in the next window, go to the root of the phone memory, go to "/Android/data/com.duoqin.promarket/files/Download".
4. On the phone start the built-in Chinese market - App Market, find any application, the size in MB of which exceeds the weight of the APK, which we are going to install, and start its download - the percentage will run on.
5. At the same time, quickly update the folder ".../com.duoqin.promarket/files/Download", there should appear .apk file with the alphanumeric name. Do not hesitate to copy the name of the downloaded file and assign it to our APK package.
6. Quickly replace with our renamed APK that which is still being downloaded from the App Market at the path "/Android/data/com.duoqin.promarket/files/Download".
7. If you were lucky and quick before the end of the download, the status in the market will go to "Installing" and may hang on this. This is normal.
8. Wait half a minute, go to the main menu and find our installed application. If nothing has been installed, or the original Chinese application appeared, then try again or substitute another distribution of this APK. If the market does not start downloading or hangs in the Installing status - clean the folder "/Android/data/com.duoqin.promarket/files/Download" (it is advisable to do this after each installed APK), and restart the phone for the another attempt.

Instructions on how to disable unnecessary built-in Chinese applications, including System Update, which constantly pops up in notifications.
1. Install (if you haven't already) and run the adb console.
2. Connect the smartphone to PC with System -> Developer options -> USB debugging turned on.
3. In the console run the command "adb devices", check the presence of the phone, its authorization status (when first connected there'll be the request for PC authorization on the phone's screen, check the box and approve).
4. Execute the command "adb shell pm disable-user --user 0 com.NAME.PACKAGE", the ok answer will be "Package com.NAME.PACKAGE new state: disabled-user".
The following firmware applications (package names listed) can be safely disabled:

com.duoqin.systemupdate com.marsqin.chat
com.duoqin.qweather com.duoqin.translator com.duoqin.inputmethod.pinyin com.duoqin.ai
com.duoqin.app.guard

5. Check the status with the command "adb shell pm list packages -d", all applications disabled in the system (not deleted!) will be shown there.
P.S. To return the application to active status - do "adb shell pm enable --user 0 com.PACKAGE NAME".
Good luck everyone!

For those, who were unlucky and updated their phones to 1.1.7 FW or later - you can try installing the market.apk from 1.0.8 FW to return the apk trick possibility back. Do cleaning cache and data of the original one before the installation.

Qin Market from 1.0.8 FW with the apk trick available.

To be able to use the numpad for old-but-good method of typing it is necessary to install manually special IME keyboard as APK - three of them tested and proofed to be working, see attached beneath. Try any. Possibly, you will need to disable the default on-screen android keyboard via adb command before.