I noticed earlier that it ALWAYS uses downloads.tf2center.com as the link.
A simpler explanation:
Lobby Create -> Check downloads.tf2center.com for map file -> If exists, do nothing. If not, do -> Get sv_downloadurl from the server -> Create a request for sv_downloadurl + /maps/<map name>.bsp.bz2 or sv_downloadurl + /maps/<map name>.bsp, If either exists download to downloads.tf2center.com after verifying it is a valid bsp file (by reading the BSP header and map name from within)