How can I write some text inside all files?

Question

linux

Hello!
Such a task that there are a lot of files, and you need to write some text everywhere inside.
How can you make it automatic?
Your options please?
Maybe write a script? But how?

Answer 1

1. Find all files
2. Open one by one
3. Write text
4. Close file
What kind of files? What type, size? More information