How indexes work in composite key and how changing the order affects the performance in Postgres 12?

CREATE TABLE public.ABC ( it_id TEXT NOT NULL, item_id varchar(50) NOT NULL, PRIMARY KEY (it_id,item_id) ); When I use above query then index scan is applied when i query only by it_id or by item_id or using both. But when I use the below then index scan is applied when I query with item_id or (item_id & it_id) . but not when I query with it_id only. CREATE TABLE public.ABC ( it_id TEXT NOT NULL, item_id varchar(50) NOT NULL, PRIMARY KEY (item_id,it_id) );