Is is there any solution to log in $session[] php with MYSQL and JSON?

Currently, I've got a problem with Session[].

When I'm log in (which is working fine). I gave Session[] "thing" but when I relocate to another window and then when I want to use it to get data from the MYSQL database.

Is there any way to work with the SESSION[] after login?

The relocated page .js:

betoltProfil();
function betoltProfil() {
    //get data from database
    var formData = new FormData();
    formData.append("funkcio", "AdoIntBe");
    fetch(baseUrl + "/ajax/AdomanyInterfacePHP.php", {
        method: "POST",
        body: formData,
    })
        .then(response => response.text())
        .then(request => {
            let szervadat = JSON.parse(request); //json data to let
            console.log(szervadat); //usually gives the <Empty string> log int the browser
            oldalEpit(szervadat);
        })
}
function oldalEpit(adatTomb) {
    console.log(asd);
}

The function php from formData:

<?php
require_once "../app/functions.php";
session_start();
if (isset($_POST["funkcio"])) {
    if ($_POST["funkcio"] == "AdoIntBe") {
        if (isset($_SESSION["userId"])) {
            AdoIntBe($conn, $_SESSION["userId"]);
        }
        // AdoIntBe($conn);
    }
}

The main function of PHP what should get the data from the MYSQL based in the SESSION[]

function AdoIntBe($conn, $userId)
{
    session_start();
//the select to Database
    $stmt = $conn->prepare("SELECT nev, leiras, email FROM adomanyszerv WHERE id =?");
    $stmt->execute([
        $userId
    ]);

    $eredmeny = $stmt->fetch(PDO::FETCH_ASSOC);
//This should give back to formData
    echo json_encode($eredmeny);
}

This is the LOGIN PHP function where I declare the SESSION[].

function beEllenoriz($AEmail, $AJelszo, $conn)
{
    $stmt = $conn->prepare("SELECT * from adomanyszerv where email =? and jelszo=?");

    $stmt->execute([
        $AEmail,
        hash("sha512", $AJelszo)
    ]);

    $row = $stmt->fetch(PDO::FETCH_ASSOC);

    if ($stmt->rowCount() == 1) {
        session_start();
        $_SESSION["userID"] = $row["id"];
        // $_SESSION["felhasz"] = true;
        echo json_encode(true);
    } else {
        echo json_encode(false);
        false;
    }
}

Overall, how to use the SESSION[] to identify the user who logs in and get the data from the database? Sorry for the heavy and hard-to-understand code!

Copyright License：
Author:「Cincennes」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/71301718/is-is-there-any-solution-to-log-in-session-php-with-mysql-and-json