You need a score card.
The Elder Gods used to own Earth. They arrived from outer space. The Great Old Ones fought the Elder Gods and caused them to lose their grip on the physical parts of our world, except for a few gateways which can be opened by their (the Elder Gods’) minions and followers. The main city of the Great Old Ones was in Antartica, but they have been extinct for several million years, being done in by their servants, the shoggoths.
Cthulhu’s minions and special servants on Earth are the Deep Ones. These are froggy-fishy humanoids. During the great whaling voyages from New England to the South Seas, some of the more degenerate sea captains brought back females of the Deep Ones as wives. Their decadent descendants are the fish people of Innsmouth. Cthulhu is presently dreaming in Ry’leh, which seems to be in modern Indonesia around the Sunda Strait.
Yog-Sothoth is his own gateway, and a rather sentient conduit which could allow others of the Elder Gods access to this plane.
Shub-Nuggurath is sort of the Earth/fertility elemental in the mythos, as Cthulhu is a water elemental and Hastur is associated with the air and interstellar space.
All books *you * are likely to encounter entitled The Necronomicon are rip-offs and drug induced sixties-era homages to Lovecraft and Anton LaVey. Don’t bother reading any of them.
And yes, the wild dogs of India are also called dholes.
Any errors and ommissions are entirely the fault of my aging memory.