Q about Connecting Flights

Hello, I have been trying to find the answer but surprisingly couldn't, so hopefully, this isn't a dumb question... Anyway I will be flying from Europe to Toronto, and almost all flights have a connection somewhere along the European west coast. Some have both legs of the flight operated by the same airline, and some are different. I was wondering if for both these types of connections, will I need to purchase extra legroom for each flight separately, even though I am purchasing through one booking, or will the seat upgrade count toward both parts of the flight? Tickets are getting expensive and just wanted to know if I should tough it out on the first leg instead of paying over $100 for extra space or if I will be covered for both parts. Thanks!