Clicks and pops

[jazzyoggy]

Apologies for  very basic questions. I am converting vinyl to digital. Old LPs often have clicks, shown as a spike on the wave pattern. I use the 'remove pops and clicks' feature and that gets rid of some, but not all.
First question - what causes clicks and pops? Is it flaws, scratches etc on the record surface or is it dirt deep in the grooves. If the latter, would vacuum cleaning solve the problem?
Second question - I assume that in a recording that was originally Mono, the L and R wave patterns would be identical. If I am right, then why do I sometimes see click/pop spikes on one channel but not on the other?

[Gale (Audacity Team)]

Obviously, clicks and pops can be caused both by dirt and by imperfections in the pressing (sometimes invisible to the naked eye). All audible imperfections will be transferred into the waveform you are recording, so it's best to use a professional cleaning machine if you have access to one to eliminate those clicks and pops due to dirt. Alternatively specialised shops may sell cartridge attachments which clean the groove ahead of the stylus, but professional cleaning is better.
Imperfections and dirt can lie on either side of the groove wall even in mono records. Not all cartridges / pre-amplifiers will actually produce identical channels, which you can demonstrate by recording a mono record in "stereo" then making both channels mono and inverting one. If the two channels really were identical, the inversion would cause them to cancel out and the result would be silence. You can of course set Audacity to record in mono if you prefer (Audio I/O tab of Preferences).