Using an example to comprehend why “safely” erasing a drive yields better results than filling it up with meaningless data

A hypothetical 1GB USB stick is full of sensitive documents/images/etc. and it is not encrypted.

The owner wishes to discard it and is aware of having to safely erase it first.

There are several tools and utilities to do this. Some can be configured to do it “faster yet less safely”, others do it “slower but more safely”.

As opposed to have it erased using all the different ways known to do this, the owner chooses to simply drag all the current items to the recycle bin and then paste one 1GB (~2-hour) black screen movie file to the USB stick.

Again, no fancy erase utilities are used. The USB stick is then discarded.

If it falls into the wrong hands, can any of the sensitive files (that filled the stick before the movie file was pasted) be retrieved?

(1) If no, why do complex hard drive erase utilities exist? Some of them feature “safe” erase procedures that take houuurs, when simply filling a soon to be discarded HD with meaningless files, can do the job?

(2) If yes, how can 2GB (movie file + sensitive files) co-exist in a 1GB stick? Seems to me like the only logical explanation is (a) the movie file was in fact less than 1GB, (b) the USB stick was secretly larger than 1GB as stated, or (c) the movie file was copy-pasted only partially and the owner did not notice.