Microblocks are a legacy feature of the previous version of Stacks that no longer exist. They were originally created as a way to improve transaction throughput, but without the functionality of Nakamoto, they never worked in practice.
Instead of microblocks, Nakamoto instead utilizes a block production structure that creates Stacks blocks at arapdi cadence.
For more on this, refer to the Nakamoto in 10 Minutes docs page.