What 'in-house' actually means
Strictly, in-house manufacture means the brand designs, manufactures, finishes, and assembles every component of the movement: the bridges, plates, gears, springs, escapement, hairspring, balance, even the screws. Almost no brand achieves full in-house this way; even Patek Philippe and A. Lange & Söhne buy hairsprings, jewels, and a few specialty components from suppliers. The standard industry definition is looser: in-house means the brand designs the movement architecture and manufactures the major components themselves.
Top in-house manufactures
Patek Philippe, A. Lange & Söhne, Audemars Piguet, Vacheron Constantin, Rolex (largest by volume), Omega, Jaeger-LeCoultre (a major movement supplier to AP, IWC, Vacheron historically), Glashütte Original, F.P. Journe, Greubel Forsey, Grand Seiko. These brands have full or nearly-full vertical integration; their movements are designed and built end-to-end.
Third-party movement suppliers
ETA (Swatch Group's movement arm, restricts non-group supply since 2003); Sellita (the non-Swatch alternative); La Joux-Perret (premium third-party, base for Czapek, Maen, others); Vaucher (Sandoz Family Foundation, makes movements for Hermès, Parmigiani, Richard Mille subcontracts); Soprod, STP, Concepto. These suppliers ship base movements that brands can specify, modify, finish, and rebrand.
What it means for a buyer
In-house brings prestige: a Lange Datograph movement is more meaningful than the same complication on a 7750 base. In-house often (but not always) brings better engineering: brands controlling their own movements iterate faster on power-reserve, accuracy, and shock resistance. Third-party CAN be excellent: a 'Top'-grade ETA 2824 in a quality finishing house outperforms a sloppy in-house movement. Don't pay 3x for in-house unless the watch has the finishing and engineering to justify it; a Tudor Black Bay 58 (in-house Kenissi) and a Hamilton Khaki Field (ETA 2824) are both excellent watches at very different prices. See wiki: in-house vs supplied movements.