Problems with lightmapping

What is the correct way to lightmap? Is there a way you can lightmap certain sections of the map bit by bit without the lightmap resetting itself everytime? I’ve tried selecting certain models to static to lightmap, then when theyre done, I unticked them, then followed those 2 steps on the next model. However this then alters the original lightmap of the previous model. Does this mean the when you lightmap, you can only lightmap the WHOLE scene at once and if you find a certain sections needs editing, youd then have to re-lightmap the WHOLE scene again? Despite the fact it takes several hours to lightmap?

PS confused why the models also receive shadows but do not make any… and yes, the settings for both “cast shadows and receive” are both ticked.

And ‘Generate UV Lightmaping’ is also enabled, and everything has mesh colliders.

Thanks for any replies

No, there is no way currently to generate partial lightmaps (without a lot of effort). It’s sad, I know, but until the feature gets added to rebake lightmaps for specific objects, this is what you’ll have to put up with.