To join multiple polylines into a single polyline, use the PEDIT command with the Multiple and Join options.

Initiate the PEDIT command:

Type PEDIT in the command line and press Enter.

Select Multiple option:

When prompted to select a polyline, type M for Multiple and press Enter.

Select all polylines:

Select all the polylines that need to be joined.

Use the Join option:

After selecting the polylines, type J for Join and press Enter.

AutoCAD will prompt for the fuzz distance. Press Enter to accept the default, which will join the polylines if their endpoints are within a small distance of each other.

Convert if necessary:

If the selected objects include lines or arcs that are not polylines, AutoCAD will ask if you want to convert them to polylines. Select Yes to convert and join them.

AutoCAD Help: Editing Polylines with PEDIT - Autodesk Knowledge Network