> Can the PIN change?
You can change it in the app, yes.
> How to issue new key if needed?
I think you’ll have to reissue your ID.
There’s also digi-ID (similar e-signature certificate on a card, but without any ID features), Mobiil-ID (e-signature on a SIM-card, no idea how it works), Smart-ID (in app, tied to secure storage in Android/iOS, cross-signed by the server which is supposed to check the device somehow) and probably something else I don’t remember. All of these are independent options, so you can, for example, revoke your Mobiil-ID if you lose your phone, and still use the your main ID card to sign things.
> You can change it in the app, yes.
Is the app tied to Google or Apple?
Nope, there’s a desktop version, too. And it’s all free/open source: https://github.com/open-eid
(Though Smart-ID is its own thing and is a fair bit more locked down, but I’ve managed to get it running on a phone without Google services IIRC.)
Wow, that is definitely more sophisticated than we have in the states. It seems like you can use it for things that one would otherwise need a notary for, that is such a timesaver.
Wow, that is nice!
How much the certificate costs and lasts?
It costs as much as your ID card costs by the government, and lasts as long as well. They are one and the same. Applying for a new ID card / national ID document in Estonia costs 35€ and the document is valid for 5 years. If you forget your PIN code, you can reset it with your PUK codes, but if you also lose your PUK codes you need to apply for a new ID card. The process for getting a new ID card from the moment you applied for it takes no more than 30 days. You can also have it fast tracked for 250€ and get it in 2 days.
But, like the parent said, you have many other options other than the physical ID-card as well. Most people these days use Mobiil-ID or SmartID, which works on your phone and even smart watch. SmartID is completely free and Mobiil-ID is tied to your phones carrier, so the cost varies, but it's a one-time set-up fee of around 5€. Mobiil-ID certificate also lasts 5 years.