new to audacity

I import an .mp3 file and
choose a very short special time interval and
fit it to the window.
I look at the left end pull-down and
see that left channel has a check mark.
Is that all that is analysed?
Which is left? Top or bottom?
I cannot change the channel.
Then I plot spectrum under Analyze.
Is that for the left channel?
On a rather pure tone, the frequency seems correct.
Please teach me.

Some links that will help you:

I presume that you are referring to the greyed out tick?
If an item is greyed out, that means that the item that is greyed out does not apply.

Plot Spectrum analyzes a mix of the two channels.