Whenever you search for any song, (in my case on YouTube Music), 2 kinds of songs usually pop-up, one the original title and the other a remastered version. I have always wondered, how the process of remastering is carried out; because remastered songs are usually shriller (from my experience) whereas the original ones are soft. Is this a side effect of remastering or is this intended? Also, when remastering a song, is the band whose song is being remastered, supposed to play the song live, or is the digital copy sufficient?
What is meant by Remastering the song? ... How [is] the process of remastering ... carried out[?]
As a starting place, the Wikipedia article on remastering gives a reasonable overview.
Is [shrillness] a side effect of remastering or is this intended?
My own experience with remastered recordings doesn't bear this out, so you might amplify this question with a specific example or even ask a separate question about a particular song of interest.
However, as suggested by the Wikipedia article above, what you're hearing could be the absence of background noise, or it could be a deliberate decision to amplify the higher pitch ranges.
Also, when remastering a song, is the band whose song is being remastered, supposed to play the song live, or is the digital copy sufficient?
Remastering only involves the previously recorded and mixed tracks. Were the band to re-record part or all of the track, this would be indicated as "rerecorded". Remastering also doesn't involve remixing, which would be indicated if done ("remixed and remastered"). Often the band isn't present at all. There are engineers who specialize in mastering/remastering, and the tapes are just sent to the engineer.