Loop and nesting on bottom side is usually top thread tension too loose (no tension) but can be bobbin thread tension issue. Be sure to thread per machine instructions.
See following Singer video... testing top thread tension.
https://www.youtube.com/watch?v=O7N4XaIV6ms
Test tension of top thread.
With needle up, presser foot up, and tension select = 4, thread machine. Then make a long slow steady pull of top thread. The thread should advance smoothly through needle eye without snagging from top spool. Now lower foot and make another long slow steady pull of thread. Again it should advance smoothly through the needle eye. Note difference no tension (foot up) and selected tension (foot down).
If no difference in foot up/down tension then something wrong with the tension selector or not threaded properly, consult machine manual.
You can do a similar test of bobbin thread tension. Make a slow steady pull of bobbin thread. It should advance smoothly from bobbin with tension similar to top thread with tension =4. If bobbin tension is similar to top thread foot up, then likely bobbin thread has escaped bobbin tension spring... reload bobbin per machine instructions.