SHORT ANSWER: They're aren't any. That's not how casting works. Companies invest a lot of money in their projects and they trust that investment to trained and experienced professionals. They're not looking for people off the street. So they don't old open auditions that anyone can go to. For most professional work you have to be invited to audition, usually rough an agent.
LONG ANSWER: How casting works is is the production company hires a casting director (CD) to find qualified actors to audition. If the CD knows of an actor they think would be good in a role (either a well known actor or a "regular" actor that the CD knows personally) they invite the actor to audition. Often the CD writes a description of the project and roles to be cast (called a "breakdown"). The make the breakdown available to licensed talent agents (not the general public). Agents review the breakdowns and submit actors for appropriate roles. To submit means to send in an actor's professional head shot and resume listing their acting training, experience, special skills and links to a show reel demonstrating the actor's previous screen work. If the CD is insterested, they contact the agent and invite the actor to audition.
In rare instances a CD may list a breakdown on an industry website like Actors Access, Casting Networks or Backstage.then an actor can "self submit" which means you send in your head shot and resume without an agent. If the CD is interested, they invite you to audition, but that's a very long shot. Plus most Hollywood productions are union productions - the breakdown will list "union" on it. This means preference is given to actors who are members of the SAG-AFTRA actor's union. And you can't just join the union you need a specific type of experience to eligible to join. Again, these are professional productions, not TV contest shows. They're not interested in taking someone off the street and making them into a star. They want actors who have demonstrated they can work at a professional level.
As for agents - you can't just hire an agent. Legitimate agents are paid a percentage of what an actor makes. NEVER pay an agent up front. Since agents only make money when an actor books a professional, paying job they are VERY picky about who they take on as clients. And they can be picky because there are alot more people wanting to be actors then there are roles. Again - agents are looking for actors who have demonstrated that they have the talent, training, experience, maturity, and perseverance to work at a professional level.
The best way to find an agent is through networking. If you can impress an acting teacher, director or someone else in the industry they may be willing to refer you to an agent. So a professional acting career starts by getting quality training and experience - not by blindly answering ads.
See it's show BUSINESS, not a lottery. People are not "discovered" and given an acting career. It's more like starting and running a business where you are the product to be marketed and sold. Be careful - there are alot of scams and rip offs that prey on people who don't understand the reality of the industry.
What you need to do (in addition to getting the quality training and experience) is to research and understand the realities of the industry. You need to understand the casting process (agents, breakdowns, casting directors); legal issues (contracts, unions, and for minors entertainment work permits and Coogan Trust accounts); marketing (headshots, resumes, show reel, website, social media); and networking. If you're a minor, your parents will have to be involved. A website with some helpful articles to help you better understand the industry: https://www.backstage.com/advice-for-act...