I have a Huawei Y7 Prime that doesn't have any released custom recoveries. Is it possible to flash or even just boot TWRP/CWM for another smartphone (eg. for Huawei P8) on my Y7 Prime? Is it possible to do this in general? and if no, is there any way to root my device without custom recovery just by using fastboot or...?!
Simple answer NO
It is not possible - recovery is device specific and unless it has been developed for your device, you are out of luck
You can try one-click apps that root, if it works for you (like Kingo root etc but be aware that they usually have their own set of problems)
Advanced answer, Yes:
If your device is Mediatek Oriented and you have it's stock ROM, the one to be flashed to the device using SP Flash tool,
- You can install the SP Tool on computer
- Extract the stock ROM
- Locate the extracted ROM when choosing a scatter file from the tool
- Don't panic here in this mode. SP Tool is by default at the Download Tab after loading the scatter.txt file since you might brick your phone
- Switch the tab to Readback
- Set the start adress as written at preloader.bin at the Download tab
- Set the stop adress as written at data at the download tab and Start the readback
- Switch your phone off and plug it on the computer(Note I'm assuming SP flash tool is well configured alongside its VCOM drivers installed but I don't want to talk about them so as to be simple. Otherwise it won't be detected)
- Let the readback process complete
- Locate the ROM dump in SP Flash Tool installation folder usually C:/Program Files
- Download Readback extractor or alternatively (use MTK_Droid_tools exe) that will convert the dump into flashable images and locate from it the source dump.(Some extractors depend on this dump while some work independently but I have to include this because one might encounter an issue)
- Start conversion.
- It will prompt you when extracting the recovery.img whether to backup the stock recovery or to generate ClockWorkMod(CWM).img instead. Chose the later. It will be called cwm-automade.img or recovery.img in the final flashable ROM
- Use this to have a more free recovery than the stock which is CWM recovery
- Flash the recovery. img from fastboot or the SP Flash tool Download tab.
Note: Do not tick preloader.bin when flashing anything otherwise you'll brick your device to a state it won't ever get alive again