Hi, senior45678.
On the majority of sites, there is normally a small square, with text, offering to save your log in, or stay logged in.
If you use a cleaner, regularly , or, worst, one which does the job automatically (Something like Ccleaner) then, doing its routine work, it will delete your "cookies". Many of these do contain your log in information.
You will often find, in browsers, (Firefox, in particular) an option to save your passwords.