It is unlikely that the cavern itself is being referred to, since caverns that last awhile are most always formed from solid rock and were not very extensible using stone-aged tools, hence「hand use to extend, inside a cavern of porn」likely referred to EXTEND, ENLARGE, EXPAND, STRETCH, LENGTHEN a client's appendage for spewing venom, or, perhaps a client's experiences. Bare-handed excavations without any tools to widen a rock-walled cave is not the obvious meaning of this glyph, either. Elements literally mean「hand use within cavern of yellow pussy emissions」as a description that unmistakably refers to a brothel. 'pussy' used here as「containing pus, a purulent wound」from sexually transmissible diseases.
　To SPREAD something「wide」to WIDEN:「both hands in use with talent to accomplish a task」during「twenty or so, numerous」occurrences in a「cavern of flashes, cavern of flickering dull yellow light, pus, and porn.」Numerous「flashes」in a wide and spacious「cavern」referring to sexual encounters. The witch-doctor shaman's service providers worked within speaking distance of each other, in those days moving from client to client, unknowingly carrying bacteria on their instruments and their unwashed hands.
　Up until a century ago or so in many hospital maternity wards around the world,「talented and skilled hands widened, spread illness」just as this glyph depicts: folks living in a city mostly were the ones using hospitals for births. Women in labor were lined in beds awaiting full dilation of the cervix prior to delivery of the infant, when a nurse「with skilled hands」would traverse the line inserting her finger(s) into the vagina of each woman in order to feel how large the cervix had dilated. After many hours of this and when fully dilated, a woman would be brought into the delivery area. Needless to say, if a single expectant mother among that day's group had a case of syphilis and/or gonorrhea (estimated quite conservatively to have been one woman in every seven, and a common occurrence today also: one in four teenage females have an STD), then every expectant mother down the line would be given a dose by the nurse's feeling fingers. Traversing the line in both directions over the course of a few hours made certain that everyone went home with more than just a new baby. Rural folks employed do-it-yerself [sic] techniques, or the pregnant woman's mother or other relative, or a midwife who worked one-on-one and was much less likely to spread an infection. More often than not the hapless husband was later blamed since a man who claimed to be loyal to his wife and to have nonetheless contracted gonorrhea or syphilis during marriage was generally not believed:「Training in obstetrics was rudimentary at best and was not compulsory for doctors until much later. In fact doctors were often the unwitting sources of infection for women in childbed, transmitting contagion from previous patients. Hospitals were places of last resort, sought only by the very poor and the desperate. The death rates in hospitals were known to be extremely high.」
　　[　https://adelaidia.sa.gov.au/subjects/19th-century-childbirth 2014-09-02　]　
　「The rise of maternity wards in hospitals meant that many women gave birth within shouting distance of each other. Doctors, in those days before germ theory, went from patient to patient, unknowingly carrying the bacteria on their instruments and their unwashed hands.」
　　[　https://www.mentalfloss.com/article/50513/historical-horror-childbirth 2023-05-18　]　 how much more so among the prostitutes?

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Chinese: enlarge
Unihan extended: expand, enlarge, stretch
EDRDG: broaden; extend; expand; enlarge