Audio editing for small section of a single track

I’m an Audacity newbie. Haven’t recorded/mixed multi-track projects since early midi-studio days a few decades ago. My issue seems like it should be quite obvious, but I’m not figuring it out. I’ve got 8 tracks recorded and I’m simply trying to adjust gain up and down as needed when a phrase or note in a particular track needs adjusting.

Working on MAC 10.14.6 with Audacity 2.4.1

You need the Envelope tool - see this page in the Audacity Manual: