Hard AF to clean...
And new ones are skanky AF! Metal grime etc...
Use super hot soapy water with sand. Soak and shake often. Repeat often...
Eventually, you'll only need to use baking soda and hot soapy water.... But at first you need strong grit to get the manufacturing crud out...
There are teirs of quality.
Glass is best. Easy to clean. Cheap to reuse from a old bottle.
Titanium, good and sturdy. Not cheap. Easy enough to clean. Much easier than stainless steel. Long term booze storage is ok. Because it's inert.
Stainless steel! Cheapest, tough. But hard to clean because it's has machine oil from manufacturing!! Hot soapy water soak required before first use. (Or waste you're booze!)
Sterling silver, while best looking, Adds a sulfur flavor to booze. Stinks from just plain old air oxidation! And require a lot of cleaning before and after each use. You can't just leave stuff in there long term.
Everything but glass is hard to clean