Bridge, soundpost, string height and strings between them decide how a fiddle plays. Three of those four are adjustable, one of them is a job for a luthier, and only one of them costs money on a regular basis.
The bridge
Not glued. It is held by string tension and it moves — usually gradually forward, pulled by tuning — until it is leaning, at which point the instrument sounds worse and the bridge is at risk of falling. Check that its back face is perpendicular to the belly and its feet sit flat. Straightening it is a two-minute job with the strings slackened slightly, and it is the single most neglected maintenance task on the instrument.
Its curvature is a real decision rather than a fixed shape: flatter brings neighbouring strings into the same plane and makes leaning between two of them easier, rounder isolates each string for a single clean line. That choice belongs to the register you play in.
The soundpost
A dowel wedged inside the body, held by friction alone, transmitting vibration between the front and back. Its position changes the instrument's voice dramatically and moving it is a luthier's job with a specific tool — do not attempt it. Its one relevant fact for a player: if the instrument is ever completely slackened, the post can fall, so never take all four strings off at once. Change them one at a time.
String height
How far the strings sit above the fingerboard at the top end. Too high and the instrument fights every note, which beginners consistently mistake for a lack of hand strength; too low and the strings buzz against the board. It is set by the bridge's height and adjusted by a luthier shaving it — cheap, quick, and transformative on an instrument that has always felt hard.
Strings
Modern synthetic-core strings are the sensible default: stable in pitch, forgiving of temperature, and warm without being dull. Steel is brighter, cheaper and louder, and is common in traditional playing for exactly those reasons. Gut is a different instrument again — lovely, unstable, and a commitment.
Change them when the tone dulls or when a winding starts to unravel, one at a time, and expect a day or two of stretching before they hold. After any change, check the bridge has not been pulled forward, then check your open strings against each other as fifths.