After transferring data from my old iMac to my new iMac, do I have to reenter all of my old logins and passwords to access sites?
You should not.
If you used Migration Assistant after setting up the new Mac, look for another User to log into.
The log in information on your Keychain app should be transferred as well. That said, it may not all be transferred. I had that problem with my transfer and Apple advised me that the Keychain app is sometimes problematic. I have a suggestion to guard against this. I created an Excel spreadsheet with all of my user names and passwords, which I periodically update and print out. I can use t for a quick reference on those sites where I choose not to have my data stored, and choose to enter it manually. Then the spreadsheet is also locked and requires a special passcode to open it. If you idon'thave your passwords store somewhere and something gets corrupted, you have a problem. Some people rely on their iCloud back up. I choose not to. (I use an external drive which is backed up manually, immediately whenever any file is changed.)
Data migration
