iMac acquiring neighbours' wifi passwords

About your Mac , make sure you have secured network see the article https://support.apple.com/en-gb/HT202068

You must have wireless network password that is same as base station passwords for airport express if used that must be saved in your router , the internet service provider router settings must be automatic .

You daughter visits neighbours , and ask their network name and wifi password and joins their network https://support.apple.com/en-us/HT202639

But when she comes back home , their network name and password must be removed from iPhone https://support.apple.com/en-in/HT208941 , since iPhone / Macs are joined on the same network at home , she knows your network name and password ( usually in families the password is shared ) and joins so their network name is visible in Mac .

In Mac system preferences > network click on advanced > wifi > remove neighbours network name ( select them and click on subtract sign ) .

Note : if your network is not secured it can be joined to open networks , a lock must appear when you click on wifi on the top menu bar on your network name see the article https://support.apple.com/en-us/HT204070 , however there are neighbours network name in the list some might be secured ( having lock in front of it , some are insecure ) , it hardly matters the point is your network must be always secure , it will join automatically https://support.apple.com/en-in/HT202480

When settings are changed , in the last always click on apply to save .

She might be sharing iCloud passwords with Mac see the article https://support.apple.com/en-in/guide/safari/ibrw12d5ed41/mac

The iCloud Keychain is set up on both iPhone and Mac both are signed in with same Apple ID and password , in your Mac open safari and its preferences and click the passwords , enter admin password to open the lock and remove the passwords .

See the section - If iCloud Keychain is on, but Safari won't save or autofill account or credit card information in the article https://support.apple.com/en-in/HT203783

Don't autofill the passwords ( uncheck the box in iPhone , not in Mac as major syncing of passwords is done from iPhone to Mac , read about private browsing in Mac https://support.apple.com/en-in/guide/safari/ibrw1069/mac

Or its the choice of your daughter or you instruct to use private browsing in the iPhone also https://support.apple.com/en-us/HT203036

Using private browsing the wifi passwords will not be auto filled in Mac keychain access .