When customer enters the details in the form fields and enter submit button then the Order email should send customer's email immediately and also Order should be created in Backend as well. Once its submitted(After email sent & order created), the success message will display on same current product page.
For this, I haved added my Custom order form on my theme's product detailed page. Form contains the fields like Name, Email, Quantity, Product Name and Price(Current Product Name & Price when customer Visits).
How to send email and create order programmatically on Product page custom form?
Check Magento default purchase order functionality, it will help to understand that thing
If you want to use own controller then check below link
hope it will help you.
please check below link
You can refer below link for Create Order and send email in magento 2,