dfs

def dfs(path, ars, combs):
    pos = len(path)
    if pos == len(ars):
        combs.append(path)
        return
    for x in ars[pos]:
        dfs(path+[x], ars, combs)

def gen_combs(ars):
    combs = []
    dfs([], ars, combs)
    return combs

ars = [[1, 2, 3], [10, 20]]
print(gen_combs(ars))