Does a Dedicated Instance guarantee physical isolation from other AWS accounts, and can it share hardware within the same account?

Yes, a Dedicated Instance guarantees physical isolation from other AWS accounts, but can share hardware with other instances in the same account

What is the primary reason to choose a Dedicated Host over a Dedicated Instance ?

To manage licenses and control physical server placement.

Are Capacity Reservations primarily for cost savings or for ensuring the availability of compute resources?

Capacity Reservations are primarily for ensuring the availability of compute resources, not for cost savings.

compare Capacity Reservations and Ris?

Use Capacity Reservations to guarantee instance availability.
Use RIs to get long-term cost savings.

Compare Dedicated Instances and Dedicated ?

Dedicated Instances: Run on hardware isolated to a single customer, but you don’t have control over the physical server.

Dedicated Hosts: Provide full control over the physical server, useful for license management and compliance.