For these questions, assume that project default settings are sampling rate = 48khz and bit depth = 24, tracks have been recorded in the project using those defaults, and these recorded tracks each have multiple clips that can be selected (highlighted) separately if desired.
Sampling rate: If only one clip in a track is selected, does resampling with procedure Tracks > Resample change the sampling rate of only the selected clip or of the whole track (including the non-selected clips in it)?
**Bit depth: Once recorded, is it possible to change the bit depth of an existing track or of a single clip within it? If so, how?
So, you’re trying to lower the quality of some clips/tracks?
The whole (selected) track gets resampled.
But when you export it may be re-sampled again to the selected Project Rate. (Of course, you won’t restore the quality if you’ have resampled to a lower sample rate.)
By default, Audacity works internally at 32-bit floating point so the bit depth of most files is increased automatically when you open/import a file. The conversion to 8, 16, or 24-bit integer and back is lossless. You can change the bit depth when you export.
If you wanted to reduce the bit dept of one track you’d have to export it separately and then re-import it. It will be converted to floating-point when you re-import it, but will have the sound of the lowered-quality.
If you want to alter the resolution of a just “one clip” you’ll have to make a separate file.