Can we Modify Existing(Standard) Report in Business Central(AL Code)?
No. You cannot create report extensions. You need to create a new report.
But you could export the standard report from NAV 2018, convert it with Txt2Al, give it a new 50000-99999 range object number and ad it to an extension.
How it work for Business Central,Means export report from NAV 2018 and open with al extension in visual studio code for Business central???
But before doing just that, get some training for learning Extensions and AL code. Or at least watch some of the many videos about it.