Not a hacked client
Check out "Minecraft Hacked / PvP /. Client Coding Tutorial" on YouTube.
In principle, it's all the same, only you leave out the hacks.
But they are all from 2016
Do you want for 1.16?
Yes would be best or for 1.14
Here
With a programming language.
Just have to recreate Minecraft as such. In such a way that it is compatible with the corresponding interface of the server.
Otherwise it does not work or it becomes a "hacked client" (which is basically nothing more than a Minecaft client, which behaves differently to the server than the official original game).
In any case you have to get MCP (Mod Coder Pack) and run decompile.bat.