You’re the defending player if a creature is attacking you or a planeswalker you control.

The Plant token blocks the attacking creature even if the block couldn’t legally be declared (for example, if the attacking creature has flying).

Putting a blocking creature onto the battlefield doesn’t trigger “Whenever a creature blocks” abilities. It also won’t check blocking restrictions, costs, or requirements.

For each unblocked creature attacking you, you must put a Plant token onto the battlefield blocking it, even if you don’t want to.

Putting a blocking creature onto the battlefield will trigger “When this creature becomes blocked by a creature” abilities. It will also trigger “When this creature becomes blocked” abilities in this case, because the attacking creature had not yet been blocked that combat.

The last ability triggers only if four or more creatures were declared as attackers during your opponents’ declare attackers step. Only creatures attacking you are counted; creatures attacking your planeswalkers are not. The creatures may be controlled by different players.