Spawn chunks are chunks found at the world spawn point which are always loaded in memory, unlike normal chunks which unload when no players are nearby.

What blocks keep chunks loaded?

Basically, it works as you have said, a long loop of hoppers will keep a chunk loaded, but the hopper chain must start in the spawn chunks. You can use chains of hoppers to basically extend the area of chunks at spawn that is kept loaded.

Are chunks always loaded in servers?

Upon connecting to a game/server the spawn chunk will always be loaded first so you have a surface in which to land on upon joining, the spawn chunk is not “Always” “loaded” but the way that it happens is it the player is in its proximity it will be loaded.

The exact rule includes chunks whose center is less than or equal to 128 blocks away from the world spawn along both axes. In the rare case where the world spawn is located at the exact center of a chunk, 17 chunks will be loaded along that axis, of which 13 activate entities.