Dear, thank you very much for your polite and constructive curiosity!
ICAO codes are simple: https://www.iata.org/en/publications/directories/code-search/ is maintained by IATA themselves, so should be reliable.

Codes like ULAS are very confusing, because they look like being ICAO codes but aren't, necessarily.  My min refrenece is an official document, unfortunately in Russian ony, but updated monthly: http://www.caiga.ru/DocAni/manual_of_4_letter_indexes/Indexes_of_Airports.pdf This gives three colums of codes: national "private", national "state" (military, dosaaf, ...) international. I take only the third column to be valid ICAO codes.

Hoping this helps,
Karel "Carlos" ADAMS
Ansiao, Portugal.