TL;DR
INPI runs the guichet unique, the website mandatory since 2023 where every business formation is filed. A platform such as LegalRapide prepares the documents from your answers and files the application for you, under a mandate. The greffe then checks the application and registers the company; Insee assigns the SIREN. 
Service-public
| Party | Role |
|---|---|
| You | Answer, sign, deposit the capital |
| Platform (agent) | Generates the documents, checks, files, follows up on correction requests |
| Guichet unique (INPI) | Receives the application and forwards it to the relevant bodies |
| Greffe | Checks and registers with the RCS |
| Insee, URSSAF, tax office | Assign the SIREN/SIRET, register for social security, set up tax records |
You can file on the guichet unique yourself: it is free, but takes longer. See going it alone or getting help.
Written by Razvan Toma, founder of LegalRapide. How we check our figures · LinkedIn