There are many different plants that produce essential oils with the scents like citronella or lemon.
The plant pictured in the OP is a scented 'Geranium' cultivar known as Pelargonium x 'Citronella', which produce essential oils in its leaves similar to that of true Citronella oil. There are also numerous other Pelargonium species and cultivars that also produce similarly scented foliage. For example, there is a separate species, Pelargonium citronellum, native to South Africa, which also produces a similar lemon scent. I grow this species or a hybrid of it in my garden.
True Citronella Oils are obtained from various species of the tropical grass genus Cymbopogon, which also contains the well-known culinary herb, Lemongrass (Cymbopogon citratus). Two of the commercially available forms of Citronella Oil are Ceylon Citronella (Cymbopogon nardus cvs.) and Java Citronella (Cymbopogon winterianus cvs.).
Another possible confusion and wrinkle of complexity in the very complex topic of essential oils, is the existence of the distantly related genus, Citronella, in the Family Cardiopteridaceae. Various species of this tropical and subtropical genus occur in Queensland, Indonesia, the Philippines, New Guinea, South America, and southern Central America. At least some of these species also produce similar cintronella-scented essential oils. The best known species is the subtropical Citronella mucronata, native to central Chile.