If you download one of these popular packs (often named things like "Ultimate Graphics," "Real Vision," or "SA_DirectX"), the results can be genuinely stunning for a 20-year-old game.
Modders have created wrappers that force the old DirectX 9 rendering engine of GTA SA to translate calls into DirectX 12. This dramatically improves CPU utilization and allows for higher draw distances and particle effects.
To get that "DirectX 30 Ultra" look, search for: