Hi. Per the READMEs (and as David just mentioned in his comment), the V2 SDKs are:
built on the AWS Common Runtime, a collection of libraries written in C to be cross-platform, high-performance, secure, and reliable.
The CPP, Python, Java and JS V2 SDKs all use the awscrt and all benefit from the reuse of common code. Since the V2 SDKs have been around for a long time already, and the V1 SDKs only get security updates, the V1 SDKs lack many of the more recent AWS IoT features compared to V2. For example (depending on the language), named shadows, Greengrass IPC, PCKS11, fleet provisioning, device defender and more.
Additionally, all of the V2 SDKs are validated against AWS IoT Core Device Advisor.
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