schengen – ETIAS mandatory or not?

ETIAS is designed to be mandatory for third-country travellers. Your first link doesn’t really contradict this. It simply says a grace period will be available before the system is mandatory.

but another page says we must have it to enter Finland and another one says mandatory to go to any Schengen country.

If you are using present tense to mean at the moment here, that’s impossible for these statements to be true. ETIAS is not in operation right now for anyone for any country.

According to official EU website, ETIAS is expected to become operational by the end of 2022. Once it is operational, a grace period of six months is provided during which the relevant member states inform travellers and improve the awareness of the new system. During this period, the use of ETIAS will be optional.

An additional six-month grace period follows. During this period, ETIAS will be mandatory but exceptions will be made for the first time a traveller crosses the external border after the first grace period has ended. This limited grace period can be extended by the Commission for another six months. (Article 83, Regulation (EU) 2018/1240)