The core problem is taxing on the fiction that are profits.
The unsavory bit is Apple maintaining at least one corporate entity that has no country of domicile, and passing cost allocations around between operating subsidiaries to take their effective tax rate to punitive rates in countries where their revenue comes from. There are similar fictional cost allocations in car manufacturing and in FMCG markets (eg: Nestle borrowing money from another subsidiary at well above banking industry interest rates, or coffee outlets paying external entities for brand licensing). There's also strange patterns on how some online vendors allocate R&D costs almost in sync with their gross profits.
The overall effective tax income from multinational companies has trended down relentlessly for 40 years in the West. The first core issue is how aggressively companies use fictional instruments and stooge third-rate economies to distort the tax paid away from the economies that provide their revenue.
The solution is to impose tax rates based on local revenue, not fictional profits.
If they do pay tax equitably, then the second core issue is how they repatriate income to the parent company's home geography. That's something for the US Govt alone.