As the aggressor, you know that based on your sizing you need villain to continue or fold X% of time. If you're bluffing pot size bet, then you're doing so because you think villain will fold more than 50% of his range now or he fold less than 50% now but will fold more than GTO on a later street.

On the river, based on your bet sizing villain will bet getting pot odds on a call. If you bet pot, villain will be getting 2:1 on a call, such that he has to be good 33%. So your range should be constructed such that the worst hand in villains calling range is indifferent to calling or folding (ie: has 33% vs your betting range).

If you read that article, it should clear stuff up too.