Simple cogs example in discord.py (tested on 2.0.1)

cogs_slash_test.py

#cogs / test.py

from discord.ext import commands

class Test(commands.Cog):
    def __init__(self, client):
        self.client = client
    
    @commands.hybrid_command()
    @commands.cooldown(1, 10, commands.BucketType.user)
    async def hello(self, ctx):
        await ctx.reply(f"Hello {ctx.author.mention}")

async def setup(client):
    await client.add_cog(Test(client))

main.py

# main.py
import discord
from discord.ext import commands
import os

class Client(commands.Bot):
    def __init__(self):
        super().__init__(
            command_prefix = commands.when_mentioned_or("&"),
            intents = discord.Intents.all(),
            help_command = commands.DefaultHelpCommand(dm_help=True)
        )
    
    async def setup_hook(self): #overwriting a handler
        print(f"\033[31mLogged in as {client.user}\033[39m")
        cogs_folder = f"{os.path.abspath(os.path.dirname(__file__))}/cogs"
        for filename in os.listdir(cogs_folder):
            if filename.endswith(".py"):
                await client.load_extension(f"cogs.{filename[:-3]}")
        await client.tree.sync()
        print("Loaded cogs")

client = Client()
client.run(os.getenv("TOKEN"))

Thanks, this example is a lot clearer.