Are airliners' cruising altitudes we know indicated or true altitudes?
The altitudes are "indicated." That is the altitude shown on the aircraft's altimeter.

In the U.S. at and above 18,000 feet (known as Flight Level [FL] 180) all aircraft altimeters are set to 29.92 in. Therefore the "indicated" altitude on the aircraft's altimeter (at and above FL180) is also "Pressure Altitude." In other parts of the world "Flight Levels" (where all altimeters are set at 29.92) varies.

"True Altitude" (actual altitude above mean sea level) is not, in an operational sense, used other than a reference for weather reports, terrain, obstacle clearance, etc. It is important, but when aircraft are flying (below FL 180) their altimeters are set to a "local" altimeter setting which adjusts the "indicated" altitude to correspond closely to "true altitude." This eliminates "true altitude" as being an operational consideration.

It sounds complicated, but it really is not.  Just a bit difficult to explain.

The range of cruising altitudes varies considerably for air carrier aircraft.  It would be fair to say that the range is generally between FL290 (about 29,000 ft. msl) to FL 400 (about 40,000 ft. msl).  This varies due to ATC operational considerations, wind speed and direction aloft, and aircraft weight and performance capabilities.  There are times when some aircraft will fly a bit lower or, if the capability exists, fly higher, say FL430.