I am new to GIS platform. I am using QGIS. I want to create lines inside map similar to border of country at a particular distance (say 100 KM). My data is in geographic coordinate system.
I would prefer to save the layer as a new file by changing its CRS into a projected one using
Right click > Export > Save Features As.... Select a suitable one for your data region. Maybe one of UTMs.
Processing Toolbox > Vector geometry > Buffer tool with negative distance value (-100000 m).