What are the overlay filesystems that appear in "mount" output when I start and stop containers with podman?

Issue

• When I start containers using podman start or podman run as the root user, filesystem mounts of type overlayfs or shm appear with paths typically in /var/lib/containers/storage:

# podman run -d --name test --rm registry.access.redhat.com/ubi8 sleep 500

# podman ps -a
CONTAINER ID  IMAGE                                   COMMAND     CREATED         STATUS         PORTS       NAMES
b584f378fda1  registry.access.redhat.com/ubi8:latest  sleep 500   22 seconds ago  Up 22 seconds              test

# mount | grep overlay
shm on /var/lib/containers/storage/overlay-containers/b584f378fda1dba79922fde7ac3370dcf61de3c2581ee339873f515eb48d8cd1/userdata/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,context="system_u:object_r:container_file_t:s0:c640,c889",size=64000k,inode64)
overlay on /var/lib/containers/storage/overlay/3459206e0c350a5c43a064444d62d8a49381e914b47a9a4872572b5c0e13a702/merged type overlay (rw,nodev,relatime,context="system_u:object_r:container_file_t:s0:c640,c889",lowerdir=/var/lib/containers/storage/overlay/l/327TUOEWRJ3W6GPCCI4WXXY2AU,upperdir=/var/lib/containers/storage/overlay/3459206e0c350a5c43a064444d62d8a49381e914b47a9a4872572b5c0e13a702/diff,workdir=/var/lib/containers/storage/overlay/3459206e0c350a5c43a064444d62d8a49381e914b47a9a4872572b5c0e13a702/work,metacopy=on,volatile)

• What are these mounted filesystems, and what is their purpose?