"The scientists analyzed the metal and metalloids inside seven types of disposable devices from three of the most popular brands. Using an instrument to activate the disposable e-cigarettes and heat the internal liquid, they created between 500 and 1,500 puffs for each device. They found:
Some devices emitted surprisingly high concentrations of elements in the vapor, including antimony and lead. Levels of chromium, nickel and antimony increased as the number of puffs increased. Most of the disposable e-cigarettes tested released markedly higher amounts of metals and metalloids into vapors than earlier, refillable vapes."
https://www.ucdavis.edu/news/disposable-e-cigarettes-more-toxic-traditional-cigarettes
IIRC, those three brands were all from the same parent company, of which was using cheap, low-quality coils/parts.
Elf Bar is manufactured by Shenzhen iMiracle Technology part of Heaven Gifts family of companies.
Esco Bar, the worst of them is made by Shenzhen Innokin Technology Co. Ltd.
Flum is manufactured by Flumigo Technology Limited, self held company.
Juul Labs spun of from Pax, 35% of it is owned by Altria aka Philip Morris who make NJOY.
All different parent companies and hardware manufacturing plants as far as I can tell.