Export

Write the model to GLB, glTF, FBX, USDZ, OBJ, STL, PLY or a parts list, and set the scale, up axis and origin the next program needs.

Steps

Click Export at the right of the top bar, or press CtrlE. A list of formats drops down.
Choose a format. The Export window opens with that format selected. You can still change it in the list on the left.
Set the options on the right. For a first try, the defaults are fine.
Press the button at the bottom, for example Export GLB.
The file arrives as a download in your browser, normally in your downloads folder. A message confirms it.

GLB: Modern binary glTF, in one file. The app calls it best for the web and Cinema 4D.
glTF: The same as GLB, written as text (JSON glTF 2.0).
FBX: Binary FBX 7.4. The app lists Blender, Maya, 3ds Max, Houdini and Unreal.
USDZ: For Apple Quick Look and Reality Composer.
OBJ: Wavefront OBJ. Universal, geometry only.
STL: Binary STL. The standard for 3D printing.
PLY: Geometry with vertex colours.
CSV: A parts list as a spreadsheet: name, colour, triangles, vertices and dimensions. No geometry is written.

Which format should I choose helps you pick. The Compare button in the Export window opens a sheet that shows what each format keeps.

Unit scale: Multiplies every size in the file. 1.0 (no change) keeps the model as it is. CAD models are usually in millimetres. If the next program works in metres, choose × 0.001 (mm → m). The list also has cm to m, m to mm, m to cm, inches to mm, mm to inches, and Custom….
Custom scale: The multiplier, when Unit scale is on Custom….
Up axis: Z-up (CAD, keep) keeps Z as the up direction. Y-up (glTF, OBJ, Maya) turns the model so Y is up, which is what those formats and programs expect.
Origin: Keep model origin leaves the model where it is. Recentre to bbox centre puts the middle of the model at 0,0,0. Centre XY, drop to floor (Z=0) centres it and stands it on the floor.
Merge all parts into one mesh: Writes the whole model as one object. Useful for 3D printing. The parts list is lost in the file.
Visible parts only: Leaves hidden parts out of the file. On by default.
Selected parts only: Writes only the parts that are selected. It overrides the switch above.
Draco compression: GLB only. Compresses the mesh data, which typically makes the file 5 to 20 times smaller. Exporting and loading are slower.
ASCII FBX: FBX only. Writes the FBX as text instead of binary. Try it if the next program cannot read the binary file.

For CSV, the options about scale, axis, origin and merging are dimmed, because no geometry is written.

The export contains the model as it is now: deleted parts are gone and reduced meshes are reduced.
It runs in solid view and switches back afterwards, so a view mode never ends up in the file.
After an export, the Optimisation report shows the size of the exported file beside the size of the file you opened.
Draco compression needs an internet connection the first time, because its encoder is one of the converters the app fetches on demand.
A very large model can be too big for a text format such as OBJ. The app then suggests GLB or STL, which are binary and much smaller.