Back then, the single-wicket format was anywhere between one against one up to five against five. Therefore, there were several possible arrangements, but the principle was that only those who are directly involved were on the field at any one time. For example, if it was one against one that meant there would be no wicketkeeper or fielders and that the bowler had to do all the fielding. In those situations, the laws of single-wicket differed slightly or maybe a little more from the longer form of the game.
Rules initially have always been the same as for any game of cricket in terms of runs and rules of getting out, etc.
The most common misconception is that Single-Wicket refers not to the stumps but to the fact that it’s a knockout competition involving individuals rather than teams. It starts with 16 players, who are drawn to play against each other in pairs.
So, if in the first round Raj (say) plays against Aman, Raj bowls an over, against Aman batting. Then they swap rounds until whoever scores the most runs in their over, and the player with the higher runs goes through to the next round. In the second round, eight players remain and so on until two players are left to contest the final. Occasionally, the semi-finals and final will comprise of three and four overs respectively.
When it comes to fielding it is done by the other players on a rotational basis. So, if player 1 plays the player 2, ten of the others will field including the wicketkeeper. The other four are usually the pair involved in the next two games or will get ready to play or kit up.