Well, when a music producer wants to sample a specific stem or instrument from a song, they often use a technique called "sampling". Sampling involves taking a small section or "sample" of the original song and using it as a building block for a new composition.

To isolate a particular stem or instrument, producers can use a process called "sampling by chopping". This involves listening to the original song and identifying the parts of the song that contain the stem or instrument they want to use. Once they have identified the section they want to use, they can use an audio editor or sampler to isolate that particular section and save it as a new audio file.

In the past, producers would use hardware samplers, like the Akai MPC, to chop and manipulate their samples. These devices allowed producers to record and manipulate audio in real time, and offered a range of editing and processing options.

Today, producers often use digital audio workstations (DAWs), like Ableton Live or FL Studio, to sample and manipulate audio. These programs allow producers to import audio files and manipulate them in a variety of ways. DAWs also offer a range of built-in effects and processing tools, making it easier than ever to isolate specific stems and instruments from a song.

Overall, whether using old hardware samplers or modern digital tools, music producers are able to sample specific parts of a song without including the rest of the track by carefully selecting and isolating the parts they want to use.

Conversely, someone could just recreate the part(s) they wish to sample digitally, physically, or otherwise.