can you really completely erase any and files and internet activities by reinstalling the Mac OS?

mikes101 wrote:

how can I completely clean my hard drive so no traces of my files or any internet activity so that not even and Lilian from mars and recover any information?

macOS is encrypted with or without FileVault....

If you have an iMac Pro or another model of Mac with an Apple T2 chip or SoC both M1 & M2, the data on your drive is already encrypted automatically.

Encrypt Mac data with FileVault - Apple Support

You can read more about old HDD cf. SDD

How to confirm that Erase in DU worked? - Apple Community

With the introduction of macOS Monterey a new feature which underscores all data is encrypted without your consent one way or the other, whether you use FileVault or not.

*ref

Erase your Mac and reset it to factory settings

ref

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Hum - follow previous advise - First

But if you are really concerned with " Lilian from mars " recovering your data - there is the Scorched Earth but not Mars method and destroy the Drive