Web SDK or REST API: When to Use for Tracking & Messaging with Optimove?

Hello everyone,
I'm integrating Optimove for web and mobile, and I've seen both Web SDK and REST API. I have a few questions:

When should I use the Steal Brainrot Game SDK, and when should I use the API?

Should I use both the SDK and the API simultaneously? Are there any conflicts?

Has anyone encountered problems fetching messages via the API? How did you resolve them?

For a system with many users, which solution is more stable and easier to scale?

I look forward to hearing from you all. Thank you!