No "loopholes". Even without mags it still lacks a magazine disconnect, which is now required.
Actually there is a couple loopholes. One requires a family transfer from parent or grandparent (and the reverse) from out of state. That is exempt from the approved list.
The other is that only semi-auto firearms are covered.
Single shot pistols and revolvers are not. There is a few single shot slides that can be fitted on the rails. Some in rifle calibers or other calibers that could not be fired through a traditional mag.
If you purchase one of those slides and have the firearm equipped with it then it is a single shot firearm, immune to the approved list.
The approved list only applies to import for sale for an FFL. Which means while it may need to be a single shot to legally be imported and sold, once owned by the purchaser it does not legally need to remain a single shot.
The gray area is then what to do with the stock slide. It would be perfectly fine to ship or sell it in a separate purchase. Selling it along with the single shot firearm though ...
Which means if a seller is willing to attach a single shot slide to a semi-auto firearm, ship it to buyers in California through an FFL, and then also have the original slides available for sale in later purchases, that is legal.
A friend out of state could make it easier. Avoid straw purchases though.
Don't engage is such activities for profit without an FFL.
Just as it is legal for someone to take an off-list (2 lists) AR, acquire a 10 round magazine, install a bullet button, and ship it to someone in California (through an FFL) as it is not an "assault weapon" and is perfectly legal.
It then has a fixed magazine, and if off-list no longer meets the definition of an assault weapon. Just like the ones people build in state.
The problem is the laws are complicated and out of state people are justifiably afraid to deal with something they do not know.
Most FFL dealers in California are not even completely up to date on the exemptions and how to make things work.
The firearm is whatever action is installed. If it has a full-auto slide it is a machinegun. If it has a semi-auto slide it is a semi-automatic. If it has a single shot upper, it is a single shot.
So if you have enough money to go buying extra single shot slides each time, and someone willing to work with you out of state, and a local in state FFL dealer that understands the law, you can get pretty much anything.
Simply having a parent or grandparent, or child or grandchild out of state makes it even easier. Those transfers are immune from the list.