I find I "tune out" because I'm concentrating on something else.

Same way I stop noticing sometimes that someone is trying to talk to me, or that I have a deadline that's drifting past, or that I haven't eaten or moved from my chair all day.

The sound doesn't go away, but I stop noticing.

I'll start noticing again if I'm trying to listen to something quiet, and realise the tinnitus is louder and slightly masking what I'm trying to hear.

Or any time I pay attention to what I can hear in the background.

I don't tend to notice the tinnitus if music is playing, or I'm talking with someone.

I agree with you that it's never gone. Though I've noticed the volume varies a lot. From quiet like the background sounds from outside where I live on a quiet day, or even quieter than the sound of breathing and my heart beating, to as loud as someone talking to me. The tones vary too. Fortunately they are usually higher frequency than what I'm listening too which reduces masking.

I've had tinnitus since I was a child, so I doubt it's due to age-related hearing loss. I don't know a cause, but back then I noticed it was louder when I was ill and stuck in bed, and actually I found it comforting then, like it sounded like my body processing as it should.