Finding them earlier than everybody else

The advertised internships are the minority and the most contested. A large share are never posted at all: they are created when a manager with too much work meets somebody plausible at the right moment. That sounds like luck and is mostly timing.

So the useful move is asking three or four months before the advertised season, in your own words, to specific people rather than to a portal. Small organisations answer far more often than large ones and the work is usually more real. One paragraph on what you can do and when you are free, sent to twenty named people, beats a hundred portal applications.