It's the fent fold / bent / drop / lean. Most likely fentanyl or carfentanyl, but almost definitely some kind of opiate (of which they're the most potent). Some of it might be neuromuscular / specifically opiate related but there's also just the practicality of it. The drug might be cut / mixed with another sedative for potency such as a benzo or barbiturate, but it also might be mixed with with a stimulant such as methamphetamine or cocaine to counteract the sedation.
You'd want to counteract the sedation either to increase enjoyment (similarly to the relaxed buzz of a caffeinated alcoholic beverage) or because they feel too much distress from being sober / unsedated but also know that they're in an unsafe area to be sedated. Even if it's not cut with a stimulant they may still be forcing themselves to stay awake either because they know they've taken enough that they might stop breathing, or because, as I said, they know they'll get mugged or otherwise attacked if they relax into the high.
Being homeless / generally in poverty is often too emotionally stressful to tolerate sober, but too unsafe to be navigated zonked, so they just put themselves into a never sleeping but never really awake haze until they either intentionally or unintentionally die or almost die or experience psychosis, in which case they either go to a morgue or to meet up with me on the psych unit.