It is said by many teachers that the Buddha taught the Mahayana Sutras. Scholars, Theravadins and others say otherwise.
Well, we know several sutras appeared so much after, some of them were created in China (like Surangama Suta, Brahma-net Sutra, etc).
Knowing this, how is explained that Buddha taught them? Was really him? Was him but just not with his Nirmanakaya form? How do you deal with authenticity? And with the chinese sutras, was Buddha too?
Also, sometimes it seems to me, that Mahayana is so complicated (comparing it to Theravada), sometimes I cannot understand anything (sometimes it seems to me that early Buddhism is much more simpler, but is just not for me). Has someone feel this too? How do you deal with it?
I sympathize with your feelings. There often discussion of this points on the Forum. It is a big topic in the subjects of 'Buddhology' or 'Buddhist Studies'. Suffice to say that one way to approach it is through books which look at the historical development of Buddhism as a movement. An early example was Buddhism: Its Essence and Development
by Edward Conze. (There are probably other and more recent titles too.) But the point is, that approach gives some framework within which to understand how the Mahayana developed as an identifiable movement, and also the way in which 'The Buddha' came to be understood as something more than, or other than, simply an historical teacher.
You can also approach through your own spiritual search and meditation practice. This will bring up questions which are often dealt with by the Sutras. Find a particular theme, teacher, or sutra with which you have a particular affiniity and just stick with it. Don't forget we're in the 'information age', all of us have access to untold volumes of information on Buddhism (and any other subject for that matter.) Quite often in earlier times, you would only have one or two sutras or teachings. After all Hui Neng was enlightened on hearing a single line from the Diamond Sutra. So, that suggests a way of approaching it thematically
rather than through a kind of 'encyclopedic' approach which tries to take in everything. Just pick a key idea, theme, or topic and work with it.