5e is more than a little vague and ambiguous and contradictory on the nature of gods and domains.
Domains are mostly a choice of the player to pick something related to their god with some suggested domains to be associated for a particular deity. When there is a suggested domain the PH only says for players to consider it.
"If you're playing a cleric or a character with the Acolyte background, decide which god your deity serves or served, and consider the deity's suggested domains when selecting your character's domain."
"DIVINE DOMAIN
Choose one domain related to your deity: Knowledge, Life, Light, Nature, Tempest, Trickery, or War. Each domain is detailed at the end of the class description, and each one provides examples of gods associated with it."
My reading of that is that a domain that is related to the deity in some way but not on the suggested list is still a valid cleric choice of domain RAW. If you wanted to think of Athena's strategic war aspects being a good fit for trickery that seems to work even with knowledge and war being the suggested domains for her. Domains are a PC power category that at least somewhat narratively fits the deity, not a defining aspect of the deity, in 5e.
Another reading of that could be that the DM, or even the suggested domains, determine what domains are related to a deity and so limits the PC clerical choices.
The DMG also says though that as a default "Gods Oversee the World. The gods are real and embody a variety of beliefs, with each god claiming dominion over an aspect of the world, such as war, forests, or the sea."
and that "In rules terms, clerics choose domains, not deities, so your world can associate domains with deities in any way you choose." So you can focus on the clerics choosing their domains part of that or on the DM associating domains with deities to determine the range of choices and get to different results.
Whether the gods are spawned by belief or powered by it as opposed to just embodying beliefs as a default seems undefined in 5e to me. The occasional reference like for vestiges seems to indicate that might be a mortal only perspective.
"Vestiges are deities who have lost nearly all their worshipers and are considered dead, from a mortal perspective. Esoteric rituals can sometimes contact these beings and draw on their latent power."