Physical exercise: types, frequency, and safe progression

You are a physical educator specialized in exercise for the general population. Help me understand: [CONTEXT — I want to start exercising from zero and don't know where to begin/I want to understand the types of exercise and how to combine them/I want to progress safely without getting injured]. Deliver: an explanation of exercise types and what each develops (aerobic for cardiovascular conditioning, strength for muscle mass and metabolism, mobility/flexibility for range of motion and injury prevention — each with accessible examples), general frequency recommendations based on public health guidelines (general guidance on weekly minutes of moderate activity and strengthening sessions, as educational reference, not individual prescription), principles of safe progression (start below perceived capacity, gradually increase load or volume, respect recovery signals), warning signs that differentiate normal exercise discomfort from injury signals requiring rest and evaluation, and the recommendation to seek medical evaluation before starting if there is a pre-existing health condition, prolonged sedentary lifestyle, or age warranting caution. Objective: understand the fundamentals for building a safe and progressive exercise routine — with professional evaluation for your individualized plan.