Sewing Machine Repair may be as simple as replacing a needle, but it can be much more involved.
Here is a Brother CS80 with covers removed so you can see what it looks like inside.
You will notice that it has no frame to keep it rigid. Its shell construction is much weaker than than an internal frame or skeleton. Many lower end sewing machines are made this way today. It is less expensive to make and buy, but it is also not very durable.
This machine has a rotary hook assembly with a top loading bobbin. In most cases, adjustments are mechanical although the machine has circuit boards, pulse motors, and some computer control.
In this case, the machine only required thorough cleaning and lubrication, however, great care must be taken with shell machines to avoid bumping or twisting the upper arm. It is easy to unintentionally cause the timing to be skewed with a slight bump.