No easy short answer to those questions.
There are two sorts of water in wood, the free water between the cells and the bound water in the cells.
Branchwood sat on the ground for any length of time the wood is likely to have absorbed ground water which will have made it soggy on the bottom side.
But it's free wood and turnable, just likely to split on you at a later date, but for practicing on, as long as you are careful, take precautions you should be okay. You might even get lucky and a nice bit of wood out of it.
Pete (bodrighywood) is our master at turning timber most of us would throw away.
The fallen birch will probably be spalted and they spalt fantastically, especially if it had a Polypore (Horseshoof) fungus on it before it fell.
If you have access to a chainsaw and are proficient with it, and you have permission, go for it.