This dataset was collected using physical ~33nm-thick sections that were cut with a diamond knife and stored (and preserved) on a carbon tape before imaging with a multi-beam scanning electron microscope.

You can get the full details in the preprint paper: https://www.biorxiv.org/content/10.1101/2021.05.29.446289v1

The latter. The tissue block is sliced first in the ATUM which collects the sections on tape. These are then mounted on silicon wafers, and imaged with the MultiSEM.

Our preprint at https://www.biorxiv.org/content/10.1101/2021.05.29.446289v1 has some more details about the process, including figures illustrating the ATUM and the tissue block.

(I'm one of the authors of the paper)