[Answered ]-How to pass multiple app's models in one views? [django]

You need to related Author and Post models to each other by ForeignKey file, like:

class Post (models.Model):
    author = models.ForeignKey(Author)

Then when you can access to your avatar through post.author.

Updated:

Updating answer after question updated.
I think you need to update your template as follows. use post.post_author.avatar to access the avatar you need:

{% for post in post_list %}
<ul>
   <li>{{ post.title }}</li> # This is works just fine
   <li>{{ post.post_author.avatar }}</li> # This does not work at all
<ul>

{% endfor %}

Considering this please note that you don’t need to search Author’s desperately in your view. So you don’t need to have following line:

context['author'] = Author.objects.all() # This is where I expect the magic is

Because when you have ForeignKey then you already have access to Author through post object.