Travel to/from Europe -- Questions re: visa and timing

Although we travel a lot, we've not been to Europe since Brexit, and we typically fly nonstop to our destination.  Not this time.  My wife & I will be flying to Rome (JFK-FRA-FCO) in September, returning via Heathrow.  Questions:  1) do we officially enter the EU in Frankfurt, even though we are not leaving the airport, or in Rome?  2) Similarly, since we change planes at LHR -- and the UK is no longer in the EU -- do we have to clear Customs or, since we're just changing planes, go straight to our next gate?  (We have a two hour layover at FRA and LHR.). Thanks in advance.