As to transition heights, altitudes and levels, each airport arrival, approach and departure chart, or even enroute low altitude maps (I used Jeppesen through my career) specified the applicable "altimeter transition". I have flown worldwide, and never used a "general reference".
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I have seen "transitions altitudes" to be as low as 3,000' QNH, and "transition levels" as low as FL 040 (or possibly even FL 035). I have seen them as high as 20,000' and FL 200, such as in the Mexico FIRs. In Canada and the USA, the "transition altitude" is (everywhere) at 18,000' QNH, and the "transition level" is translated by the wording "lowest usable level" which is FL 180 for QNH at or above 1013.2/29.92 and 500 feet higher (FL 185 or sometimes even FL 190) depending how low the local QNH happens to be below standard.
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Then, many locations publish "transition level by ATC", and is often on ATIS. I recall Hong Kong had a transition level at some FL 120 or 130 with a certain QNH value, and with a lower QNH value, to be 1,000 feet higher.
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My approach/landing briefing to my crew, in the case of "transition level by ATC" not yet received, was to assume it to be 1,000 feet above the "transition altitude published", thereby providing a "transition layer" of some 1,000 feet for most cases. That kept us out of trouble until the actual "transition level" was received on ATIS or by ATC.
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For you in Europe, pay attention to the "transition level" for the Geneva or Milano area, particularly with low QNH and below standard temperatures. It may be darned close to the "granitoculumus" called "the Alps". Same story applies to cross the Andes between Argentina and Chile.
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Happy contrails