Don't want to write this by hand? GodCraft sets minecraft:movement for you with a switch.Get GodCraftOverview
minecraft:movement is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Sets the base walking speed of the entity. Most land mobs sit around 0.2 to 0.3; set it to 0 for a mob that should never move by itself.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:movement": {}
}