1.How do I replicate a specific section in after and I remain him on the same timeline (same row)?
You don't. AE's timeline uses layers and splitting anything always splits the layer into new layers. It simply works different from editing programs.
2.Can I set up a second-long video all the time loop will play back without a break?how?
File --> Footage Interpretation --> Main. Set the number of loops in the dialog. Alternately, use time-remapping with two simple keyframes and add a loopOut("cycle") expression to the time-remapping property.