I am trying to understand how postgres works for a select query with 100 million records.

Imagine a table called users which has 100 million records.

if 2 different client(A and B) issue select * from users, what does the postgres do? Does it create a snapshot of the entire table for every client? i read it here a SELECT query sees a snapshot of the database as of the instant the query begins to run. What will happen when the client is slow to consume the data…Will it not affect any update to the users table in the meantime?