Hope these folks can help you further. https://www.malaysianchinesekitchen.com/ingredients/vegetables/

Link to varieties of Chinese mustard. The B. juncea var. rugosa Bailey looks close to yours(?). Again I’m only speculating. http://ongzi-secretgarden.blogspot.com/2013/04/genetic-diversity-of-chinese-vegetable.html

I can’t tell for certain with out tasting it. I could only speculate for you whether these plants are in the lettuce genus (Lactuca) perhaps the Romaine type (L. Sativa L. var. longifolia) or Brassica species e.g. leaf mustard/mustard greens. A taste test might narrow down quickly, leaf mustard has a spicy taste compare to lettuce. You can harvest leafy greens anytime depends on your preference. Also you could let some of these grow to maturity (flowering stage), their blossoms will tell whether they are Lactuca or Brassica. Check out these include links for images of Romaine lettuce & Chinese leaf mustard. You could try to get a more accurate ID with the folks on the last included link. Best wishes! http://www.botanic.jp/plants-ka/coslet.htm