The terms social anxiety and shyness are sometimes confused – many different people use the words in many different contexts. However:

The main symptoms that distinguish Social Anxiety Disorder (SAD) from shyness are:
-The intensity of the fear and anxiety felt
-The impairment of everyday functioning that it may cause in a person’s life
-The level of avoidance and safety behaviours

Shyness: People who are shy may feel socially quiet or uncomfortable, but after being in a situation for a while they may become more at ease. It might feel awkward talking to people, or perhaps they prefer to keep themselves to themselves, but it is more of a personality trait and doesn’t result in the same levels of anxiety as those associated with the disorder.

Social Anxiety Disorder: A more extreme fear of particular social/public situations. Sufferers will often fear being embarrassed or worry about being negatively evaluated by others. Anticipatory anxiety and avoidance are common, and individuals may endure social situations with a high level of discomfort. The scope of the anxiety may be focused on specific settings or generalised to almost any social/public encounter.

Although many people experience some form of shyness in new situations or with unfamiliar people, it is generally tolerable and understandable in the context. For those with social anxiety disorder, particular situations are very challenging. The anxiety feelings can be intense and out of all proportion to the public or social situation involved. Without self-help or therapy (see what the NHS offers and how to make progress) they can become part of a cycle of self-consciousness, distress and avoidance. See the social anxiety maintenance cycle.

Please also see these pages :
Difference between social anxiety and performance anxiety

www.aboutsocialanxiety.com/social-anxiety-vs-shyness

www.aboutsocialanxiety.com/introvert-vs-extrovert

www.calmclinic.com/social-anxiety/shyness

www.socialanxietyinstitute.org/shyness-or-social-anxiety-disorder

www.verywellmind.com/difference-between-shyness-and-social-anxiety-disorder