How would you generate multiple buffers from multiple polylines all not connected and can't be connected since they aren't related? In other words, is there a geoprocessing tool that let's you select one polyline and generate multiple buffers instead of having to select one polyline after another ?
There is no need to select features to buffer them, just remove any selection and use
Buffer (Analysis)—ArcGIS Pro | Documentation
or one of the other buffer types in that topic