This page contains some information for developers about the SQL standard.
First, there is some information in the PostgreSQL documentation:
Getting the standard
The standard needs to be purchased from ISO or a member organization.
At ISO, search for "9075" (the number of the standard), e.g., https://www.iso.org/search.html?q=9075.
For most work related to PostgreSQL, you only need part 2 (that is, ISO/IEC 9075-2), also called "SQL/Foundation". The other parts are for special areas of functionality, not all of which are implemented in PostgreSQL. (See the PostgreSQL documentation linked above for more details on the parts.)
Participating in the development of the standard
The SQL standard is developed by ISO/IEC JTC1 SC32 WG3, which stands for joint technical committee 1, subcommittee 32, working group 3. The committee and subcommittee levels are mostly for administrative work, so don't worry about that initially. The real work happens in the working group.
To be able to participate in the working group, you need to join a national body that is a member of ISO, and join the local committee or group that mirrors the international group. They will then delegate you to the international group. The appropriate national body is ideally the one in the country where you live, or in some cases alternatively in the country where your employer is. Each national body sets their own rules about this. Start by checking the web site of the national body most relevant to you and contact them. (It might just work to ask something like "I want to join ISO/IEC JTC1 SC32 WG3, can you get me in touch with the local mirror committee".)
When you join, what you get is a login to the content management system, and then you can start reading documents and see the meeting invitations and go to meetings.
SC32 has a web page at ISO: https://www.iso.org/committee/45342.html. If you have questions about how to get in touch with the working group or the corresponding national body, you can contact the committee manager listed there.
Of course, PostgreSQL community members who are WG3 participants can also help you.