How is the data from the form processed when two users access it simultaneously?

Question

PHP

Hello!
The server has a PHP + HTML form. After filling in the required fields, the Calculate button is pressed. Further. An exe file is launched that uses the data from the form and calculates the result, which is written to a file with a unique name. The exe file is finished. Further, a link to the file is sent to the specified Email. Explain what happens if two/many users fill out the form at the same time and press the calculate button? Is a queue created or copies of processes created?
Where is there information about this? Thank you very much in advance!