What is the difference between polkadot and Avalanche?

Could you please elaborate on the fundamental differences between Polkadot and Avalanche? I'm particularly interested in understanding how their respective architectures, scalability solutions, and security mechanisms differ. Additionally, I'd like to know about the use cases and communities surrounding these two projects. How do they compare in terms of adoption and overall ecosystem growth? As a cryptocurrency enthusiast, I'm keen to grasp the nuances that distinguish Polkadot from Avalanche. Thank you for your insights!