Get all of these particular items from your local armorer. Yes. they've been going to Advanced Armorer School, obtained their Diploma Of Advanced Armorer-Type Things IV. Well, some of them have, good luck with yours! They're all top-level trades, so place down a lot of Blast Furnaces in your village, grow lots of villagers, and trade up all the armorers. Patience (as with most things) is key!
Once you have traded for one of these beasts, you need to do a thing before it will do anything other than walk around, slowly. Hop in, get a netherite bar and hold it, interact with your machine. This initializes the mech and it will now fly (look up and jump), drill down (look down), and mine horizontally. It also provides the user night vision and underwater breathing. These armorers have thought of everything!

Disclaimer: This is an unofficial app for Minecraft Pocket Edition. This app is not affiliated in any way with Mojang AB. The name, brand and assets are the property of Mojang AB or its respectful owner.