With most password managers you can just import the passwords stored in your browser. For the rest: there are several ways to find out where you signed up. One of them is to search in your e-mail for phrases like ‘account activation’ ‘welcome’, ‘verify e-mail’, ‘registration’ etc. Do this also in your local language. This way, I was able to resolve 99% of my accounts and even find the ones I created more than a decade ago.
Second step is to delete them or put them in your password manager (plus update with a strong password and enable 2FA if possible).