I try to create a chair following the youtube tutorial. Could you guys tell me why the object seems like broken, not a smooth rectangle after I use solidify. Also, the same happen again when I use the modifier in order Solidify > Subdivision and then Bridge Edge Loop to connect two faces. Below images show the problems and the file link is also added Blender file. Appreciated if you can help me out of this trouble.
Your face orientation is causing this problem. To see face orientation you can check Face Orientation under Viewport Overlays tab. Blue is outside. Red is inside.
To solve this select your object and toggle to edit mode. Press a to select all. Then press Shift+n to Recalculate Outside. This fill flip normals that are inside-out. You can also select Mesh tab and from Normals select Recalculate Outside.