USB pendrive/disk formatting clarification
Not sure of a quicker way than that, my only advice is to not try to find direct correlations between Windows and Mac functions, some stuff is just different, for example the Eject options are similarly more work to find on Windows.
Erase will remove all files from the target volume, often via the expedient method of just creating new Table of Contents areas whilst technically leaving the old content there, just not addressable without specialist utilities.  Erase Free Space will actually hunt down files which should no longer exist, and rather than just remove the pointers to those file, will actually overwrite them with junk data (all zeroes for example)
Files in the Trash are often per volume, so delete a file on a USB stick, and it goes in the Trash area on that USB stick.  The file still exists, so the space is still allocated. Empty the trash to free up the space.  This (similar to above) will actually just remove the pointer to the file, unless you choose "secure erase trash" which will genuinely remove the file contents also.