Multiple teams working on the same product: how are they Sprints aligned with each other?

• Are they necessarily synchronized (that is to say: they start at the same day and have the same length)?

I believe the need is for all involved Scrum teams to be able to regularly sync up their work around each increment.   That can result in teams working at the same sprint cadence, or at different cadences that allow for this to happen (i.e - one team working in 2-week sprints, another in 3-week sprints, syncing up every 6 weeks).

• If one of the teams' Sprint is cancelled, what happens with others (this question make sense if the Sprints are synchronized)?

It is difficult for me to envision a scenario where multiple Development Teams are all working from the same Product Backlog, but the Product Owner makes the call to cancel one of the Development Team's sprints because the Sprint Goal no longer makes sense.



That said, I believe this is definitely an "outlier" situation (very uncommon, per the Scrum Guide).   The affected team should regroup immediately to determine what can be salvaged from the canceled sprint, and re-plan the remainder of the sprint to coincide with either the original time box, or a sync point with other Development Teams working from the same backlog.

• Are they necessarily synchronized (that is to say: they start at the same day and have the same length)?

In order for a Nexus Sprint to be planned and an integrated increment to be delivered, might it just be necessary for individual team Sprints to align at a common boundary at least every month?

• If one of the teams' Sprint is cancelled, what happens with others (this question make sense if the Sprints are synchronized)?

The Scrum Teams in the Nexus would try to replan their work in such a way that the Nexus Sprint Goal can still be met.

• Are they necessarily synchronized (that is to say: they start at the same day and have the same length)?

Once the overall work for the Nexus is understood, Nexus Sprint Planning continues with each Scrum Team performing their own separate Sprint Planning. The Scrum Teams should continue to share newly found dependencies with other Scrum Teams in the Nexus. Nexus Sprint Planning is complete when each Scrum Team has finished their individual Sprint Planning events - Nexus Guide  

So it would be wise to synchronize them? To ensure the above protocol for scaled model with multiple teams of 3 or more which should follow Nexus?

If one of the teams' Sprint is cancelled, what happens with others (this question make sense if the Sprints are synchronized)?

If one of the Sprints get cancelled according to the NEXUS format it would be wise to re-plan the current work (like drop , adjust or add any work that is dependent on the cancelled sprint work) while making sure the Nexus Sprint Goal can still be met.

Also I am not sure if answers to above mentioned questions differ whether we work inside a Nexus or leveraging just a Scrum without a Nexus?

NEXUS is required for 3 or more team. So if you just have two teams going either way would not have much impact on the overall planning for the combined product delivered from both teams. IMHO NEXUS and its rules can be optional for 2 teams.