"You can train your solders basic skills."
How can i do this? Need building? Where is this option?
Think in this way. OpenXcom is a engine that have some improvements, but it is basically "vanilla" Xcom we all know and love. It let you run the game, and yes, some mods too, but only in the framework of the original game mechanism.
OXCE and OXCE+ are modifications, addons, improvements, or whatever you want to name it, that let you do some other stuff that in "vanilla" OXC is not possible. This is, it add some other mechanisms that are not in the normal game. But, you need a mod that use those mechanisms for these to work. OXCE and OXCE+ are not mods for the game, but for the engine.
One example of this is the possibility to add pilots to the ships. It is not a vainilla feature, and is very hard that you will see it in OpenXcom in the short place (Or ever). But OXCE+ add this possibility. The thing here is the word possibility. If you´re a modder, you can use it in your mod, but if you don´t want, you don´t. It is the same with OXCE and training soldiers.
I hope this help to clarify some misunderstandings.