To mirror the arm to the other side of the body, first select the entire arm and deselect anything you do not want mirrored. You can select individual objects by holding Shift while clicking, or use a marquee selection to select the entire arm at once. If you accidentally select an object you don't want, hold Shift and click that object again to deselect it.
Once the correct objects are selected, go to Mesh > Combine.
With the newly combined object selected, go to Mesh > Mirror ☐.
In the Attribute Editor, keep the World coordinate setting and turn off Combine with Original. This will create a mirrored copy of the arm on the opposite side of the body.
For the leg, the process is the same, but you will mirror it in the opposite direction. Select all the leg components, making sure only the objects you want to mirror are selected. Go to Mesh > Combine. With the combined leg selected, choose Mesh > Mirror ☐. In the Attribute Editor, keep the World coordinate setting, turn off Combine with Original, and set the mirror direction to the opposite axis from the arm. Change the Mirror Direction to positive . This will create a mirrored copy of the leg on the other side of the body.