Python: Find exact matching words from 2+ txt files files

Question

data1 = open("a.txt", 'r').readline().strip().split(',')
data2 = open("b.txt", 'r').readline().strip().split(',')
res = list((set(data1) - set(data2)).union(set(data2) - set(data1)))

how can I do the above but with multiple txt files? they are comma seperated, and in lines. I want to find matching words given data1.

data1 == data2

data1 == data3

data1 == data4

...

From:compare two file and find matching words in python

with open("file1") as f1,open("file2") as f2:
    words=set(line.strip() for line in f1)   #create a set of words from dictionary file

    #why sets? sets provide an O(1) lookup, so overall complexity is O(N)

    #now loop over each line of other file (word, freq file)
    for line in f2:
        word,freq=line.split()   #fetch word,freq 
        if word in words:        #if word is found in words set then print it
            print word
output the same words found in both files:

apple

The output in my case should be words found in those files that are also found in data1... any idea is much appreciated!