How can I retreive and display all posts that belong to a certain category

I have 3 tables

user: id, username..

subreddits: id, name, created_at..

posts: id, title, link, user_id, subreddit_id

The problem is, I am fetching the id of the subreddit/category manually while I need to be fetching it dynamically. How can I achieve that?

This is the show() method in SubredditController.php

    public function show(Subreddit $subreddit)
        {
    
            $posts = Subreddit::findOrFail(3)->posts()->get();
    
            return view('subreddit/show')->with('subreddit', $subreddit)
                                        ->with('posts', $posts);
        }

And this is Subreddit Model

    class Subreddit extends Model
    {
        protected $fillable = [
            'name',
            'description'
        ];
    
        public function user() {
            return $this->belongsTo('App\User');
        }
    
        public function posts() {
            return $this->hasMany('App\Post');
        }
    }

Also. am I doing this right? Is there a better way?