A Sprint is never completed ahead of time. A Sprint is a timebox - it has a fixed start and a fixed end. In terms of events, the Sprint Planning is the first event and the Sprint Retrospective is the last. When it comes to finishing early, there are two possibilities. One possibility is that the team has met its Sprint Goal prior to the end of the timebox. The second is that the team has completed all of the work selected for the Sprint Backlog before the end of the timebox.
The team meeting the Sprint Goal before the end of the timebox isn't an uncommon occurrence. I'd even say that this is expected. I would encourage a team to craft the goal such that implementation of about 60-75%, with a maximum of about 85%, of the Product Backlog Items selected for the Sprint will lead to the completion of the goal. This helps the team to consistently meet their goal and regularly deliver valuable increments Sprint-over-Sprint, while still handling any unexpected events that arise in the Sprint. When the Sprint Goal is complete, the team can continue working on their Sprint Backlog for the remainder of the Sprint.
If the team has completed all of the identified work in the Sprint Backlog, there are a few options. They can continue on paying down technical debt or working on technical enablement. This would include refactoring, adding additional automated tests, improving the test and deployment tools and processes. The team can also spend additional time on refinement for potential upcoming work. There could be opportunities for cross-training to improve the team's cross-functional nature. Pulling in more work is an option, but it would be one of the last options, and I would want anything started to be completed by the end of the Sprint.
I would also point out that there is no "next Sprint" to pull work from. A Sprint is not planned until the Sprint Planning session. The reason for this is that the Sprint Review can change the order of the Product Backlog. As the stakeholders review the work that was done and synchronize with the team on changes to the environment, there could be new work added or existing work reordered.