If the Spanish came to the Americas earlier than the brittish and they interacted for a longer period of time with the amerindians (having way more time to infect the native population with old world diseases), why are more amerindians in Mexico, Central and South America than North America if supposedly "90% of amerindians died from diseases such as smallpox" during the colonization?
Shouldn't there be an similar amount/percentage of amerindians in North, Central and South America, since they all died at the same rate?
If the Spanish came to the Americas earlier than the brittish and they interacted for a longer period of time with the amerindians
That's part of the answer right there. Because Spanish contact happened earlier, there has been more time for indigenous populations to bounce back in Spanish-controlled regions.
In Anglo-America, indigenous populations fell slower and more recently. The nadir happened in the early 20th Century and indigenous population numbers have been rising since then.
Another thing to consider though is that the Spanish initially targeted dense population centers like Mesoamerica and the Andes. Not only have these areas had longer to bounce back, they started out with more people in the first place.
The third thing to consider is that the Anglo-American colonization pattern differed from their Spanish counterparts. The Spanish tended to usurp indigenous authorities but keep the common people in place. The Anglo-American strategy generally pushed Native communities further and further into the periphery while settling their own people onto the newly vacated lands. So in Anglo-American territory there has been a far great push to outright supplant Native populations with Euroamerican ones.