general contractor (civil engineering and construction)
Refers to a company or business that undertakes both civil engineering and building construction work.
That general contractor undertook both the bridge and the building.
A local general contractor is in charge of the road construction.
建設業者 is a broader term for any construction company, while 土建業者 specifically implies involvement in both civil engineering (土) and building construction (建).
ゼネコン (general contractor) is a more common, modern term, often referring to large-scale contractors. 土建業者 can sound slightly more traditional or regional.
Compound of 土 (do, 'earth, civil engineering'), 建 (ken, 'building, construction'), and 業者 (gyōsha, 'business, contractor').