[2019] Questions about when new lines are introduced

Heyo!

Welcome to the game, great to have you onboard.

So during the early access stage of line releases, ship missions can be obtained through various ways (depending on the event). When completing the ship mission, you will receive the ship as a "special ship". The ship is as it would be on its full release however from it you are not able to progress further in the line from it until the line is fully released.

For example; with your Monteccoli, you can not research further from her on the line until the full release of the line in update 8.10.

When update 8.10 is released, you will be able to progress from the Monteccoli down the line as normal.

Fem,

Those aren't really premiums, but (currently) non-researchable special ships. Earlier they were displayed as premiums (as a simplification "what is not researchable — is premium"), but actually as of recently (current update) there's a separate category with a separate icon: https://worldofwarships.com/en/news/game-updates/update-089-italian-cruisers/#technical-improvements

The difference might be subtle, but what's important — is that it allows to filter (or filter out) those special ships in the carousel.

I feel other points were already answered sufficiently.