Milano Porta Garibaldi (Milano / Milan / Mailand)

Currently the only international express trains from Italy to France are these daily TGV trains, which travel from Milan to Paris via Turin/Torino.

The overnight train service from Milano to Paris has been withdrawn.

Arona on the shores of Lake Maggiore is close enough to Milano to be part of the city's regional train network, which is operated by Trenord and not by Treniatlia.
The only options for taking a train from Milano to Arona are therefore the Regionale (R) services operated by Trenord.

Though a complication is that most of the trains depart from Milano's secondary station, Porta Garibaldi, with fewer departures also available from the city's main rail station, Milano Centrale.
So if you will be buying tickets last minute at the station (you can't save by booking in advance) it's best to look up the train times on the Trenord website before setting off for the station
There are gaps of more than an hour between departures from both Porta Garibaldi and Centrale stations

Despite the reduced Eurostar timetable in response to the pandemic, it is still not only possible, but entirely feasible to make a journey by train during the day from Milano to London (as long as you won't be travelling on a Saturday).
The timings allow plenty of time to cross Paris , so you won't be rushing from the onward connection against the clock.

Though something to look out for is that the trains to Paris will depart from Porta Garibaldi station in Milano and not at the city's main station, Milano Centrale.

What isn't now an option is taking an overnight train from Milano and then a Eurostar on from Paris, as the night train between Italy and Paris has now been withdrawn.

The overnight train service between Paris and Milano has been sadly withdrawn so travelling by the day on a TGV through the Alps is now the only option for a direct Paris to Milan rail journey.

Though when looking up this journey on the ticket agents, you'll likely see other daytime options with connections between trains in Basel, Geneva, Lausanne or Zurich.
If you'll be happy to change trains and the connections are easily timed (more than 30 mins ideally) and the price is competitive then these other options are definitely worth considering.

The journey from any of those Swiss cities to Milan will be more scenic than this route and you'll also have the convenience of arriving at the main station, Milano Centrale.
These direct TGVs arrive at the city's secondary station, Porta Garibaldi.

If you want to travel direct from München/Munich to Milano/Milan/Mailand by train you will need to travel overnight by Nightjet.

By day a connection between trains is required in Verona Porta Nuova station, but the end-to-end journey will be faster; and it's also a stunning train ride.

Despite the reduced Eurostar timetable in response to the pandemic, it is still not only possible, but entirely feasible to make a journey by train during the day from London to Milan.
The timings allow plenty of time to cross Paris (if you book separate tickets per train), so you won't be rushing from the onward connection against the clock.

Though something to look out for is that the train from Paris will arrive at Porta Garibaldi station in Milano and not at the city's main station, Milano Centrale.
The train will also arrive in Milano after the night trains south to Lecce via Bari and to Sicily will have departed.
Though the train from Paris will call in Turin/Torino, where a connection is available into a night train on to Roma and Napoli.

What isn't now an option is taking the Eurostar to Paris and then the overnight train to Milano, as the night train between Paris and Italy has now been withdrawn.