removing part of a model to replace with another (equipment)

Hello i've been modeling for a while but i'm quite new to Unity

i'm using Blender to model for Unity and i was looking into how to remove part of a body model(the raw body) and replacing it with a model for Equipment

to put it simply, if you look into DarkSouls's equipment, if you.. say equip armor, if you peek into the sleeves you can see that the Body's model under the armor is removed (i assume it's to save performance)

since we're trying to make a mobile game, i figured saving performance would be quite an important task, also, it would help remove clipping from the body model if the clothes are quite tight or loos

i'm not the one in charge of programming, but i figured there might be some things i need to prepare in terms of the models in order to make them viable, but do let me know how to program those in so i can point this to my collaborator!

the question is

1. (modeling) is it necessary to have specific/special model settings to get this working? if so, what is it and how to do it? (i can model)

2. (programming) how to program those in? / how to make Unity remove part of a model and replace it with another?(overlay and remove anything under it, even if i need to set up vertex groups in a special way)