What affects Pokemon spawn rates?

Spawn rates are affected by:

• Individual Pokemon. Some Pokemon spawn much more frequently than others, for example Rattata is a relatively common spawn compared to Snorlax.
• Weather. If a weather boosts that type, Pokemon of that type are more likely to spawn
• Nests. If there is a nest for a particular Pokemon, that Pokemon is much more likely to spawn until the nest is re-rolled. Here's a list of Pokemon that can have nests.
• Location of the spawn point. It's believed that certain spawn points are more likely to spawn Pokemon of certain types. Spawn points near water being more likely to spawn Water-Pokemon near water is the most well-known example. Beyond water though, the rest is based on observation and speculation with questionable evidence. For example at a creek near my old house pre-Gen 2, Poison-type Pokemon apart from Grimer and Koffing (rare Poison-types) were common spawns but Water-types weren't, but I wouldn't say that's the case for every creek.
• Regionals. They only spawn in certain countries
• Events. Obviously events can make certain Pokemon much more likely to appear, for example Community Day