Amplification

I discovered how to amplify an entire stereo file. Is it possible to amplify only one channel (left side, in my case)?

You can do that with the “Channel Mixer” Nyquist plug-in. It is available here: Missing features - Audacity Support
Installation instructions are here: https://manual.audacityteam.org/man/installing_effect_generator_and_analyzer_plug_ins_on_windows.html#nyquist_install

Sometimes, Normalizing stereo channels independently can fix problems. See: Normalize

Here’s still another option: Splitting and Joining Stereo Tracks