What is the correct way to pack records into an Eloquent collection?

Question

Laravel

There are two tables: categories and posts. The post is tied to its category (category_id).
The category model uses hasMany(Post::class). The Post model uses belongsTo(Category::class)
In a loop, I can access category->posts() and display posts from that category.
There was a task to pack posts back into categories.
For example, the input has a list of posts
Post1 category_id 1
Post2 category_id 1
Post3 category_id 1
Post4 category_id 4
Post5 category_id 7
and so on.
How to get a collection of three categories (id 1, 4, 7) at the output of these five posts, so that later in the cycle when calling category->posts() these posts are displayed each in its own category?
Can this be done using Eloquent itself?