Now lots of people prefers video editing in smartphone, rather than computer. But, there is a common misconception is good gaming phone can edit good quality video. But that's absolutely not. Now I will mention video editing smartphone in every price catagory. And I will specially highlight kinemaster app.
Flagship smartphone
Every flagship smartphone can be used for heavy video editing or high-end video editing. But iphone will be the best option. Specially iphone 11, 12 or iPad will be the best in this category. But if you prefer android smartphone then you should go with OnePlus or samsung mobile. Things you have to keep in mind, the processor performance and optimization. Best is to use Snapdragon processor phone or you can also use Exynos flagship processor only. But don't use MediaTek processor for better experience and high-end editing. I will recommend only kinemaster video editor app for Android users for advance editing. I will not recommend any other app for advance editing becouse they are not optimise for Android os.
Premium midrange smartphone
Basically here I will recommend smartphone under 40k to 20k. As earlier I had said, try to prefer Snapdragon processor phone and avoid samsung smartphone under this category. You can buy OnePlus 7 or mi smartphone or realme smartphone with Snapdragon processor only.
Budget smartphones
Here, I will mention smartphone under 20k to 10k. In this category you could go with Redmi note 9 pro/pro max , poco X2/3, poco M2 pro, realme 6 pro/ 7 pro etc. with Snapdragon processor. In this price segment a lots of MediaTek processor smartphone are found but don't buy those smartphone even whatever the an tu tu score or whatever the gaming capable. MediaTek processor are not optimise for video editing. You will not able edit 4k or 2k video with MediaTek processor.
Entry level smartphone
In this category you can buy any good processor smartphone with Snapdragon processor. You will get upto Fhd editing option. But I will recommend you to prefer sd 665 processor, if possible.