Am I obliged to let my employee see everything practical I do with patients, or should I conceal some things from her to prevent her from imitating me and to avoid future harm in light of competition, knowing that I will answer any theoretical questions she has? And is this considered concealing knowledge?

The basic principle is that an employee is not obligated to share their expertise with colleagues, unless withholding it would harm the work. Retaining this expertise is not considered the forbidden concealment of knowledge. However, it is preferable to share this expertise with those who can benefit from it, as this falls under the category of ihsan (doing good/excellence).