Well, I can't tell you what information Intuit actually keeps, but I can explain why the different login procedure. Banks felt that the old system was not secure enough. The new system means that Intuit does not handle any of your login information. Instead that goes directly to your financial institution along with the information that the request is coming from Intuit and what specific information is being requested--current filing year tax forms. This system is hopefully more secure and cuts down on possibilities for fraud.
Note that if your login includes accounts that are not relevant to your current tax return in progress, you need to select only the accounts that are relevant. Also, you may find that some documents don't transfer and you need to verify that all the documents needed have transferred. Case in point: TD Ameritrade transitioned to Schwab in 2023. The data from TD Ameritrade prior to the transition will not transfer from Schwab, even though it is available (through a special procedure) on the Schwab website.
