IMPORTRANGE on Google Sheets, one has to "connect sheets" as follows:
Where can I check which sheets are connected, and what exactly (what access rights etc.) this "connection" implies?
You are not able to check for "connection" anywhere in Google Sheets. Such a feature does not exist at the moment. As for the "what access rights" part: a spreadsheet that is connected can be read (scrapped) with
IMPORTRANGE function and this can also be exploited to gain direct access (not ownership) to the whole spreadsheet.
As it was mentioned on the other answer Google doesn't offer a place that include a list of the connected spreadsheets but you could create automatically that list by using Google Apps Script.
The access rights that the connection implies is read only and it's granted to the account that authorized the access. This connection will work while the authorizer has access to the spreadsheet, if they are removed then IMPORTRANGE will stop to import the data until another user authorize it again.