How do I make form action attribute go to themes function.php

Make the action the same page or to the categories page:

<div>
   <form name="formSenda" method="POST" action="../categories/">
      <input type="text" placeholder="name" id="nameSenda" name="nameSenda">
      <input type="email" placeholder="emailSenda" id="emailSenda" name="emailSenda">
      <input type="submit" value="Check" name="submitSenda">
   </form>
</div>

Then in your functions file, check to see if the form has been submitted and then process the info:

if($_REQUEST['submitSenda'] == 'Check') {

   $nameSenda = $_POST['nameSenda'];
   $emailSenda = $_POST['emailSenda'];
   if(!empty($nameSenda) && !empty($emailSenda)){
     $message = "Hi $nameSenda, is your email $emailSenda?";
     $headers = "From Fly Gadgets \r\n";
     mail($emailSenda, "Invoice", $message, $headers);
   }
}

I’m glad that worked.

First, before going any further, you should read up on validating and sanitizing input data to avoid creating a security problem with your site.
https://codex.wordpress.org/Validating_Sanitizing_and_Escaping_User_Data

For the wp_mail:
Have you sent a test email from the plugin? Does that work?

Do you get any errors on the screen, in the php error log or have you tried turning on wp_debug in config and seeing what it says?

If it’s not executing wp_mail, it’s likely that it can’t find it, or it doesn’t have permission to run it. See what the error logs and messages say.

PS: remember to turn off debug before taking the site live.

I think you may have your headers set incorrectly.

Try and see if this line works instead of the mail line:
wp_mail(“yourname@email.com”, “Invoice”, “Test”);

Change yourname@email.com to your actual email. If this works, then you know the wp_mail function is working.

Read this about using headers:
https://developer.wordpress.org/reference/functions/wp_mail/#using-headers-to-set-from-cc-and-bcc-parameters

There is no correct way to call wp-load.php. Your code should not load WP. If you want to use wp_mail you need to let WP load your code and hook into the appropriate action to execute your code when WP loads. As I said, this code should not be in the theme’s functions.php file.