The bill does say what it covers:

Age-restricted social media platform (ARSMP) is defined at proposed section 63C. This section outlines the scope of the proposed obligation to be introduced, while also providing the Minister with flexibility to specify which platforms are or are not covered by the provisions through making legislative rules.

The definition of ARSMP draws on the existing definitions in the Act, in particular electronic service (section 5). This broad definition, with some exclusions, covers:

• a service that allows end‑users to access material using a carriage service
• a service that delivers material to persons having equipment appropriate for receiving that material, where the delivery of the service is by means of a carriage service.

Under proposed section 63C, an ARSMP would be an electronic service which satisfies the following conditions:

• the sole purpose, or a significant purpose, of the service is to enable online social interaction between 2 or more end-users[2]
• the service allows end-users to link to, or interact with, some or all of the other end-users
• the service allows end-users to post material on the service
• such other conditions (if any) as are set out in the legislative rules.

Further, the Explanatory Memorandum (EM) states that, ‘in the first instance’, the Government proposes to make legislative rules to exclude messaging apps, online gaming services and ‘[s]ervices with the primary purpose of supporting the health and education of end-users’ from the definition of ARSMPs

Broad as usual, but basically from my understanding any website thats primary purpose is user to user communication of some form, unless its a messaging, health, or gaming orientated service. Also the minister can directly exclude services as they see fit.

proposed subsections 63C(6) and (7) provide that the Minister may specify that a digital service is out of scope of the definition of ARSMP

How the system is actually going to work seems to be all up to the eSafety Commissioner and this bill seems to just to say, your job is to make this happen with XYZ requirements, and here are the penalties if someone doesn't comply once it's implemented.