Supplements can still provide certain benefits even if you do not exercise regularly. For example protein powder can help you meet your daily protein needs and vitamin supplements can help correct nutritional deficiencies. However supplements alone will not build significant muscle because muscle growth requires resistance training and adequate recovery.
If your goal is to build muscle then taking supplements without exercising will not give you the results you want. Your body needs a training stimulus to encourage muscles to grow. Supplements can support your nutrition but they cannot replace weight training or a balanced diet.
Protein powder can be useful even when you are not going to the gym because your body needs protein every day. Protein supports muscle maintenance and other important bodily functions. However drinking protein shakes without exercising will not automatically make your muscles bigger.
Creatine can increase the amount of creatine stored in your muscles even if you do not work out. However the strength and performance benefits are most noticeable when creatine is combined with resistance training. Taking creatine alone is not a shortcut to building muscle.
Some supplements can help support general health even when you are not physically active. For example a vitamin or mineral supplement may help address a deficiency. However taking extra vitamins when you already get enough does not necessarily provide additional benefits.
Taking BCAAs without exercising may not provide much additional benefit if you already consume enough high-quality protein. Protein-rich foods and complete protein supplements provide the essential amino acids your body needs. Your overall daily nutrition matters more than buying several different products.
Taking supplements without exercise is not necessarily harmful but it depends on the ingredients and dosage. Some products contain excessive amounts of certain vitamins stimulants or other active ingredients. Always check the label and avoid taking multiple products with overlapping ingredients.
Supplements may help correct nutritional deficiencies even if you do not exercise. For example someone with a confirmed vitamin deficiency may benefit from an appropriate supplement under professional guidance. This is different from using a bodybuilding product to increase muscle size without training.