Farmer in India brings along solar panel to charge phones for extra cash

Quickly running some numbers. A square foot of solar panel can be expected to output 15 W. Assuming you convert the 12--15 V of the panel down the the 5 V required by USB, those 15 W can give you a current of 3 A. A "fast charger" outputs 2.1 A, so 3 is more than enough.

Why this powerbank solarpanels then need days to get charged by solar? There are Powerbanks you need to have 24/7 under the sun and they even then need long as hell to charge. I think they are just trash.. They don't rly working good. Some of them don't even let them charge by solar because their panel seems to be thaaatt bad.

How big are the panels on those powerbanks? Also, this 15 W/m² assumes normal incidence; it's only half that on average if you don't track the sun but let the panel flat on the floor. Then there's a problem with heat: the efficiency of solar panels decreases when it gets warm; so if the panel is glued to the powerbank the heat may have difficulties dissipating. Finally, solar powerbanks are probably made from cheap crap.

well, to see the size you can just go to amazon and search "powerbank solar" ad you will then find a huge ton of powerbanks which have solarpanels on it. they are on the whole front / back of it, so normaly it should work "okay" imho. but it don't.