The vast majority of routes and operators are covered. Main exceptions are some low-cost operators, such as Italo, Ouigo, Iryo. And some mountain & private railways.

For a full list check here;

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/railway-companies

And a better summary here:

https://www.seat61.com/interrail-and-eurail-reservations.htm

In the UK, almost everything’s included, the main exceptions being Heathrow Express and tubes/ trams/ metros.

The coverage works by train companies but yes the vast majority of routes are included. The main thing to be aware of is mandatory reservations on high-speed trains and some cross-border routes (depending on the country/company). They cost extra and somewhat limit your flexibility as you need to book in advance.

I’d suggest reading https://www.seat61.com/how-to-use-a-eurail-pass.htm and https://www.seat61.com/interrail-and-eurail-reservations.htm

Now more specific :

Venice - Orta San Giulio : 13€ reservation required on the Frecciarossa high-speed train to Milan and then reservation-free trains via Novara.

Orta San Giulio - Zermatt : fully reservation-free as the mandatory reservation on EC trains only apply as far as Domodossola (coming from Milan), where you’d board it.

The train to Zermatt is fully included but unfortunately you don’t get any discount on the cogwheel train to Gornergrat or other cable cars in the area.

Great Britain : really good value there as last-minute tickets tend to be expensive! Reservations are free of charge, optional but recommended on busy routes like London - Manchester.

Questions welcome ! :)