GOVERNMENT & MEDIA RIDICULE

Exactly. It has been my experience that most light duty rivets have rounded heads, not flat heads flush with the 2 surfaces they are bonding, with through holes in both of the surface materials; typically 2 sheets of flat metal.
Simple rivets use a hollowed sleeve that drops into a through hole in the material surfaces.
Then a pin is inserted, with a pan head at one end of the pin.
A rivet gun compresses the pan head into the sleeve, expanding the diameter of the sleeve outward, in a friction fit up against the inner part of the through-hole of both surfaces that are to be bonded together, and this application is not meant to be disassembled, except for deconstruction.
I have no experience with aviation riveting, though, and may be mistaken in this instance.

A cross notch Philips head is for a tool insertion to induce rotational torque.
Straight slot is simpler, and a Thompson is a Canadian square “hole”
in the top of the screw head. The Thompson driver (screw-“driver”) bit is the male portion of the square, where the screwlhead has the female square cavity.

Rotational torque for screws / threaded products:
For the simple minded:
Righty-tighty; Lefty-loosie.

Machine screws for industry come in two types of thread.
Course threaded and fine thread.
Coarse thread means fewer threads per inch of length of screw;
Fine thread means more threads per inch of screw by comparison.

If you go to a hardware store, a typical small machine screw size is
10-24.
Significance: It’s a size 10 (diameter) screw, with thread spacing machined to use 24 threads per inch of length.
-Even if the size 10-24 spec machines screw is 1/2 inch long, 1 inch long, two inches long; whatever.
A finer thread example would be a 10-32, where there are more threads per inch, regardless the total length of the screw.
If < a full one inch of length, then the thread count is fractional.
I.e., a size 10-24 machine screw that is 1/2 inch long will have 12 threads
showing.

How are the threaded holes made? …that a machine screw goes into?
A cutting tool called a tap thread is used, that will slowly cut a thread size to match the planned screw size to be used.

The thicknesses of the metal pieces to be bolted together will be one factor to determine the diameter size of the machine screw, and the thread size; either coarse thread or fine thread.

Example.
For a machine screw sized 1/4-20, again,
1/4 represents the diameter of the screw, with 20 threads per inch of length.

So, to drill and tap a 1/4-20 hole into a metal surface, two metal cutting tools are used.
1- A metal drill bit smaller than 1/4 inch in diameter, is used to bore the hole in a target surface.
2. A tap bit, sized for creating a 1/4-20 thread has carving edges at the tip of the bit, and gradually increases the diameter of the carving edge and threads, progressively up the stem of the tap bit.

A tap bit must be rotated slowly, and with moderate pressure, and used with plenty of lubricating oil while carving out the thread-hole.

If forced with too much pressure and rotational torque to try and cut out the surface metal hole too quickly, the tap bit can break, if it is also dull after too many uses.
If it snaps and gets stuck in the through hole drilled < 1/4 inch diameter,
that brittle, broken tap bit is a SOB to extract.

Any other machine tool veteran can jump in and correct me anywhere,
or add their 2 cents to the lesson.

Machine screws can be more reliable with holding power, if either
blue or red Loc-Tite liquid is applied to the screw threads, prior to assembly.
Better get it right, though.
Undoing a machine screw that used Loc Tite can be quite a challenge in some instances.
Removable, but not always easy.
Heat application can help.

Lastly, a flush mounted screw head is shaped like a 'funnel' head, where the top surface, when tightened aligns flatly in parallel with the surface the hole is drilled into. 'Flush'

A pan-head machine screw has a slightly bulbous head to it, resembling a miniature cookie, if viewed from the side.
A pan-head will stick out above the material surface it is screwed into, and is the most common type of machine screw, containing much finer threads than wood screws.
Soft material: coarse thread.
Hard material: fine thread
is the general rule of thumb.

A drywall screw is a very coarse threaded example of a flush mount, also called a counter-sunk, where none of the screw head protrudes above the surface material.

Merit-based hiring will, too.