
der Steg
jetty / dock
A structure that leads from the shore into the water and serves as a mooring place for boats or as a walkway.
Die Segelboote sind am hölzernen Steg am See festgemacht.
The sailboats are tied up at the wooden jetty by the lake.
jetty / footbridge
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jetty / dock
A structure that leads from the shore into the water and serves as a mooring place for boats or as a walkway.
Die Segelboote sind am hölzernen Steg am See festgemacht.
The sailboats are tied up at the wooden jetty by the lake.

footbridge
A narrow, simple bridge, often made of wood, that leads over a stream, a ditch, or marshy ground.
Ein wackeliger Steg führte über den reißenden Gebirgsbach.
A rickety footbridge led over the raging mountain stream.

bridge
A narrow connecting or supporting component, for example the part of glasses that sits on the nose, or the part of a string instrument over which the strings are stretched.
Der Steg seiner Brille war gebrochen, deshalb rutschte sie ständig von der Nase.
The bridge of his glasses was broken, so they kept slipping off his nose.
You can recognize the meaning of 'Steg' from the context. If it is about water and boats, it means a mooring place. If it is about crossing a stream or ditch, it means a small bridge. With glasses or instruments, it is the connecting piece.
Wir saßen auf dem Steg und ließen die Füße ins Wasser baumeln.
We sat on the jetty and let our feet dangle in the water.
When the 'Steg' (jetty/dock) is a mooring place, you use 'am Steg' for the position and 'auf dem Steg' for the surface. Movement there is often expressed with 'zum Steg'.
Das Boot liegt sicher am Steg.
The boat is safely moored at the jetty.
In the meaning of a small bridge, 'Steg' is almost always used with the preposition 'über' and the accusative to describe crossing.
Sei vorsichtig, wenn du über den alten Steg gehst.
Be careful when you walk over the old footbridge.
bridge
Brücke is the general term for a structure that crosses an obstacle. A Steg is usually a narrow, simple bridge, often made of wood and crossing a stream, ditch, or marshy ground. Use Steg for this simple type of structure and Brücke as the general term.

Die Wanderer überquerten den Bach über einen schmalen Holzsteg.
The hikers crossed the stream over a narrow wooden footbridge.
Die Wanderer überquerten den Bach über eine Brücke.
The hikers crossed the stream over a bridge.