Delete cell phone data completely?

I own a Huawei P30 Lite which I would like to sell now. Since I have also used this as a work cell phone in the meantime, confidential data is available there. I first deleted all data normally, but since I heard that the data could still be restored, I downloaded the ishredder app from the Playstore, which has relatively good ratings for data destruction.

I had the cell phone overwritten twice with the simplest method, I think it is only overwritten with 0. Then I downloaded the diskdigger app and tried to restore the data.

I could still see some photos and many looked like the file was damaged after being overwritten, so maybe you can only see half the picture with the other almost garnix stripes of purple.

Are the data not automatically encrypted on the Huawei p30 Lite so that no one can retrieve them after weeks of settings?

What other options are there to securely delete the data from my cell phone since, as I said, they are important documents.

Reset to default.

Settings - System and Updates - Reset

and then the part is virtually virgin again

But data are then deleted but they are only released to be overwritten so can you restore them?

If you think THAT, then dismantle the cell phone, remove the memory chip and destroy it, then you are 100% safe

Everyone resets the cell phone before he sells it and when someone buys the cell phone and really has nothing better to do with his life except to look and try out whether he can still get the data of the previous owner… He's really to be regretted