how do eSIMs actually work for travel?

I’m traveling soon and keep seeing people recommend eSIMs instead of buying a local SIM card, especially services like Holafly.

I get the basic idea that it lets you activate data without swapping physical SIMs, but I’m still confused about how it works in real life. Do you install it before the trip and it just starts working when you land? And does it replace your normal number or can you still use your regular SIM at the same time?

For people who’ve used Holafly or similar services, was it actually worth it compared to getting a local SIM?
Any hidden downsides I should know before relying on it for a trip?

I’d love to hear what worked best for you while traveling.