Italics in LaTeX (e. g. when using Tikz)

\itshape is the command for italics (this and \bfseries are unrelated to tikz)

The font styles in your document are defined by the class document or by a package or by you in the preamble or in your document. The font style used in the environment tikzpicture is the font style defined when you enter in the picture. You can change the style in the part of text in a node with font= ...

Dimensions like 1em will be with respect to the font used outside the node (usually the font that was in force when the picture started)

Like David wrote finally the font styles are defined in latex and Tikz do not change anything.