Aligning two tracks

I am new to Audacity. I am operating a MacMini using 10.15.5 Catalina
The Audacity version is 2.4.2

I have two mp3 tracks, one of a rhythm guitar, one of bass. I recorded the bass track listening and playing to the rhythm guitar track.
Each track has a different amount of lead time before starting. The bass track starts two beats after the rhythm guitar starts.

Problem 1. Alignment. With both tracks on the screen I cannot work with only one. If I wish to add silence, or delete opening noise, both tracks are effected. On a call with a friend to help, he brought two independent tracks (not my two) in to his screen, and he was able to work with one while the other remained untouched, and then the other way around.

Is there a setting I do not see that does not allow the separate work on one of the tracks?

Problem 2. Tempo. Even though I recorded the bass by listening to the other track, the playback is not the same tempo. How can that happen?

My eventual goal is to get these two tracks in sync (and in the same tempo), and then add the third track which is the melody and a solo section. These three tracks will become the audio track for a video presentation. The mp3s that I am editing with on Audacity were originally recorded with video on the camera roll of either an iphone or ipad, and saved as .mov

Could the original .mov file converted to mp3 be part of the problem? Will I have a similar problem going from mp3 to imovie to use the audio track to edit the video from?

Everyone - - thank you in advance for all the guidance and assistance in getting my first project together!

select the track you want to modify …

then any effects will only be applied to that track.

BTW Using the time-shift tool is easier than padding with silence …

It sounds like you have Sync-lock Tracks selected. If so, de-select it.
– Bill