Writing fake data would be a time consuming and boring task for a developer. Because of that, this process should be automated. However, the data generated should reminiscent the production data. That means, you shouldn’t create a string of random letters if the value should be a name of a user, or an email.
For generating seed data, you can use faker.js and write a function that will generate random data every time you run it (Code Snippet - Generating random data - Faker).
When generating data, make sure that Unique entities in DB are generated as unique.