Very generally speaking everyone who is on a Section will be sectioned because (as a result of their mental illness/disorder) they pose either a risk to themselves or others, or are at risk of their health deteriorating further without the right intervention.
The most commonly used sections (in hospital) are Section 2 and Section 3 of the Mental Health Act.
Again speaking very basically/generally, Section 2 is for assessement and lasts up to 28 days. In theory during that time a diagnosis will be made, (or indeed a decision made that the person does not have a mental illness.)
The person then may go on to be regraded to a Section 3 for treatment. (or the person may become more aware of their situation and chose to accept some sort of treatment voluntarily).
A Section 3 initially lasts for up to 6 months. (this can be extended but certain conditions have to be met). The person will be detained for treatment, which in some circumstances can be enforced, e.g. by injection but this is fairly rare.
People who already have a diagnosed mental illness and need sectioning obviously don't need assessment so would be admitted under Section 3.
I stress that this is a very basic overview. Hope it is helpful.
Your friend will only have items removed that may pose a risk to himself or other patients. e.g. razor blades. Some hospitals have stricter policies than others.