You need to have a Xiaomi account that has been given permission for unlocking device.
Download MiFlash from https://www.miui.com/unlock/index.html
Download the crDroid ROM for Redmi Note 7 https://crdroid.net/lavender/9
Download firmware update from [1]
Download NikGapps from https://sourceforge.net/projects/nikgapps/files/Releases/NikGapps-T/. Core variant is enough.
Download OrangeFox recovery for Redmi Note 7 https://orangefox.download/en/device/lavender
- crDroid version 9.8 introduce dynamic partition. We need to use OrangeFox because is the one that support dynamic partition since https://orangefox.download/release/64e67545531cf4e269b8aaa3.
Download Magisk from https://github.com/topjohnwu/Magisk/releases
Connect the phone to your laptop and open MiFlash to unlock your device following the instruction.
Reboot into bootloader mode
Execute the command to detect your device fastboot.exe devices
Flash OrangeFox recovery from bootloader with fastboot.exe flash recovery recovery.img
Reboot into recovery mode fastboot.exe reboot
Wipe Dalvik, cache, system, vendor from recovery
Copy the downloaded files to your phone in recovery mode
Flash firmware update from recovery
Flash crDroid from recovery
Reboot into system once and reboot again into recovery
Flash NikGapps from recovery
Boot into system
Install Magisk-XX.apk
Follow this instruction to complete Magisk installation https://topjohnwu.github.io/Magisk/install.html
Install F-Droid
Install Aurora Store from F-Droid
Install Gboard
Install Chrome from Aurora Store
Install 腾讯应用宝 from Chrome

Flash crDroid ROM for Redmi Note 7

Reference