They convert Planet OSM and Natural Earth OSM dumps into the storage format used by their geospatial DB [1] and then distribute them using P2P file storage tools like IPFS [3] or hyperdrive [4].
Because those storage tools make use of content addressing [5] and clever chunking [6], you can download only changes to the (converted) OSM dump.
Because those tools are inherently P2P-focused and come with pluggable transports, the data representing the changes in the (converted) OSM dump don't have to be sent via the internet. For example, Mapeo [7] allows syncing "local" changes to OSM via USB sticks [8].
[1] https://github.com/peermaps/eyros [2] https://github.com/peermaps/data [3] https://ipfs.io/#why [4] https://hypercore-protocol.org/guides/modules/hyperdrive/ [5] https://docs.ipfs.tech/concepts/how-ipfs-works/#content-addr... [6] https://blog.gopheracademy.com/advent-2018/split-data-with-c... [7] https://www.digital-democracy.org/mapeo/ [8] https://docs.mapeo.app/complete-reference-guide/mapeo-deskto...