I can understand that my question probably sounds very stupid. This is not the first tube I've installed 🤣But reading the instructions tubes must be inflated a bit before installed.Now take a look at those images 1-
2-
Those are two different tubes inflated just a bit for the installation itself.One is completely flat against the rim and the tyre installation can be done easily.The other one is almost impossible since the tube folds on itself making it impossible to install correctly without pinching. Note that both tubes are newer, fresh from package.