I have ~16 different passwords that I regularly use for various sites/accounts/etc. that I enter manually (without saves/password managers). Those are what I use probably on a bi-weekly basis. They do rotate and change every few months. There are a few that I just have saved in a text file that I copy pasta because I doubt that if anyone hacked me they would be interested in logging into those accounts. If anyone hacked those sites, they probably wouldn't be able to tell if my password was plain text or encrypted.
I have 4 main passwords and probably a couple of others I don't remember. All 4 passwords are long phrases and have special characters/numbers/capitals to make each password unique. They also have unique words in them, usually vaguely related to the purpose of the account. I don't store them anywhere, I remember them by only using variations of one password for one thing. All of the PMYA accounts, for example, all use the same password phrase with differing variations. If I don't remember a password, I will get it within a couple more tries even though they're all different.
So I kind of do what your mum does, just with more autism.
Edit: The only thing I do write down is wallet words, because fuck remembering that.