Making list from elements from each level of a nested list

I have a list:

cleaner = [['Re:', '31_300323', 'RE777']]

I want to check if another list contains same value, I write:

any('31_300323' in sl for sl in cleaner)

,
and get "True", but if I write:

suka = []
for h in cleaner:
    if any(h in sl for sl in cleaner):
        suka.append(h)

the emptry list remains empty. Why? Thank You

Let's iterate your code,
when you write "for h in cleaner" it means that h is every element in cleaner so the first and only element in cleaner is ['Re:', '31_300323', 'RE777'] so now h would be the list i mentioned.
Now let us see your if condition in this condition you are checking if h in sl where sl is ['Re:', '31_300323', 'RE777'] by the same logic in above paragraph since sl doesn't have ['Re:', '31_300323', 'RE777'] as its element therefore it would return false and therefore your list is empty.
You are welcome

flatten function: used to flatten the nested_list

Code:

def flatten(lst):
    """Flattens a nested list of any depth."""
    flattened = []
    for item in lst:
        if isinstance(item, list):
            flattened.extend(flatten(item))
        else:
            flattened.append(item)
    return flattened

cleaner=[['Re:', '31_300323', 'RE777','21_110123'],[[['26_100822']]]]
zajavki=['21_220223', '21_110123', '23_200123', '26_100822', '25_260123', '31_300323']

# Flatten the nested list
cleaner_flat = flatten(cleaner)
suko=[]
for word in cleaner_flat:
    if word in zajavki:
        suko.append(word)
print(suko) # ['31_300323','21_110123','26_100822']

Note* If you don't want duplicates you can use data structure set()

Let's say you known the depth of the nested_list than you don't need to flatten the nested list

Code:

cleaner=[['Re:', '31_300323', 'RE777'],['Ze:', '23_200123', 'RE778'],['De:', '21_220223', 'RE779']]
zajavki=['21_220223', '21_110123', '23_200123', '26_100822', '25_260123', '31_300323']

suko=[]

for sublist in cleaner:
    for check in sublist:
        if check in zajavki:
            suko.append(check)
print(suko) #['31_300323', '23_200123', '21_220223']