On disk a partition is a directory, <topic>-<partition>, in each replica's log.dirs. Kafka 129 only appends to it and deletes only whole old files. Appends are sequential writes, which disks handle at full bandwidth; they go to the operating system's page cache, with durability coming from replication rather than fsync; and consumers of recent data are served from that cache, on unencrypted connections straight from file to socket (sendfile).
The producer builds record batches that the broker stores unchanged and consumers receive as the same bytes, so a batch is compressed once, by the producer, and decompressed only by consumers (Compression Codecs).