Anisotropic optical and electronic properties in layer-engineered <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">HfSe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi mathvarian

The layer-engineering-dependent electronic and optical properties of two-dimensional 1T-$\mathrm{HfSe}{}_{2}$/1T-$\mathrm{SnSe}{}_{2}$ van der Waals (vdW) heterostructure are investigated by employing first-principle density functional theory. In the independent particle approximation framework, the twist-free heterobilayer exhibits indirect-band-gap features similar to constituent monolayers, however at particular rotational angles $\ensuremath{\theta}={16.10}^{\ensuremath{\circ}}$ and ${\phantom{\rule{4pt}{0ex}}43.90}^{\ensuremath{\circ}}$, the twisted heterobilayers demonstrate direct-band-gap features. The direct band gap in twisted unit cells arises from atomic registry and corrugation effects, beyond simple zone folding. Furthermore, the emergence of a direct band gap supports allowed optical transitions, consistent with optical selection rules, reinforcing the link between band-gap character and optical activity. Semiconducting states to metallic phase transitions emerge under the applications of electric fields. The heterobilayer shows strong optical response in the visible range, while twisting induces both red- and blueshift features, with additional infrared peaks emerging under fields of ${E}_{\mathrm{field}}=0.4\phantom{\rule{4pt}{0ex}}\text{V}/\text{\AA{}}$ and ${E}_{\mathrm{field}}=0.5\phantom{\rule{4pt}{0ex}}\text{V}/\text{\AA{}}$. Bethe--Salpeter equation calculations reveal previously unreported multiple bright excitons in the twist-free heterostructure. We find contribution of both interlayer and interband electronic transitions for the observed strongest excitonic peak. These findings highlight the tunable light--matter interactions in layered two-dimensional materials, opening avenues for engineering direction-dependent optical functionalities in next-generation optoelectronic devices.