Alias vs Indices Query vs Multi-index

Hi all,

I want to differentiate the three things alias,indices query and multi-index,

for example :

Alias : we can create alias for multiple index and search with name of alias
Indices Query : here we can specify various indices and make search over them
Multi-index : Here we also specify multiple indexes and then make seach

from my point of view,. all these three points to same things