On which page of the Player's Handbook are the rules for object interactions located ? I mean the whole "one free object interaction" thing ? Where is it stated ??
You are looking for "Other Activity on Your Turn" on page 190 of the Player's Handbook and page 73 of the Basic Rules pdf:
You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.
This section has below it a table of example object interactions (unfortunately placed below the Movement and Position header).
Additionally, the "Use an Object" action is described on page 193 (pg. 75 of BR):
You normally interact with an object while doing something else, such as when you draw a sword as part of an attack. When an object requires your action for its use, you take the Use an Object action. This action is also useful when you want to interact with more than one object on your turn.
the Use and Object action is described briefly in the PHB, page 193.
It describes the action as "When an object requires your action for its use, you take the Use an Object action" and assumes that using an object is usually part of another action, like attacking with a sword or casting as spell with a components pouch.
Specific rules for each item will be described on the item entry.
However, this counts as an Action unless otherwise stated.