How to successfully make phone device storage to be clean completely

[ABCbits]

What you need is military grade tools like SysTools Data Erasure Software or Advik Data Wipe Tool to get the job done, they are both capable of overwriting data multiple times to ensure that it can never be recovered, good luck.

Both tool you mentioned are paid software with free demo. Do you actually use any of those? Is it better than free one?

I don't know why everyone is only suggesting factory reset. Actually, when you do a factory reset, your mobile's storage is basically formatted, and if you want, you can get that data back with data recovery software. I think the best way is to first do a factory reset, then fill your mobile's storage with some junk data. And then do a factory reset again. And if you do this process 2-3 times, hopefully your storage will be completely clean. Even if someone want, they can't recover data with complex data recovery software.

As i stated previously, Android require encryption these days. So you'll need decryption key (or exploit known bug) to decrypt any data you could restore. But when you perform factory reset. Android actually delete encryption key[1] and then use new one.

[1] https://android.stackexchange.com/a/249587

[Synchronice]

But that is not the main solution, he wants to know how he can make sure he deletes all the data stored on the phone. I know by deleting the data from the phone, the data has not been deleted on the UFS or the eMMC of the phone.

He needs how he can permanently delete all the data on his phone.

If he wants to ensure that all data will be deleted without the possibility of recover, then honestly the best thing that he can do is to destroy the storage but when I say destroy, I mean complete destruction, like burning it in a fire, which will make it completely impossible to recover because it will be melted.
By the way, for any average and above average person, it's completely enough and more than enough to do the following: Encrypt your device, then perform a factory reset. Then overwrite the data yourself, i.e. download random data from the internet, like install apps, games & download movies, images and so on. Then do a factory reset again (your device should be encrypted). Do it 3 times in a row and there is no way anyone will be able to have access over your friend's old data. I don't think that someone will spend billions of dollars and the time to try to recover his data

[satscraper]

But that is not the main solution, he wants to know how he can make sure he deletes all the data stored on the phone. I know by deleting the data from the phone, the data has not been deleted on the UFS or the eMMC of the phone.

He needs how he can permanently delete all the data on his phone.

Android has an "Erase all data" option, although its location may vary depending on the phone model. On my phone the path is: Additional settings==>Back up and reset==>Reset phone==>Erase all data.

However, even performing the factory reset does not guarantee complete security, especially if the phone was manufactured by Chinese company. China relevant phones are subjects of concerns about potential surveillance features that may be integrated at the hardware level. So to be on the safe side I would destroy phically the chip ^{on board} relevant for all kind of communication with outside world.