Don't want to write this by hand? GodCraft sets minecraft:is_collidable for you with a switch.Get GodCraftOverview
minecraft:is_collidable is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Makes other mobs bump into this mob instead of passing through it, so they can even stand on it. Both mobs need minecraft:collision_box, and it works best when this mob stays still.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:is_collidable": {}
}