Okay, progressing slowly!
What are these extras/collectibles like
dog tags all about - meaningful for the game somehow?I am asking coz I see no explanations inside the game ... everyone meant to know everything except me

Dog tags, vehicle keys, emails and collectables open up stuff in the extras section on the main menu. I think the dog tags give you pictures of particular characters in the storyline, vehicle keys show pictures of some of NYC's cars and service vehicles, as well the as in-game Marine, Cell and Ceph ones. Emails actually show the behind-the-scenes conversations (In email text format) between some of the higher up characters in the game. And lastly the collectables show things like vista shots and concept art for the areas you visit.
Here's a tip on the aliens, many people don't use it because it doesn't do a lot of damage but the K-VOLT is surprisingly effective on the Ceph aliens, even the shielded and heavy ones. After all it does murder on suit energy, the same tech that the aliens use. You can get them from special CELL operatives, you'll know when you encounter one when they start firing electrical pellets and zeroes out your suit.
well as far as i know
Kvolt is at the end of the campaign? and the reason why Kvolt is effective against aliens is cus theyre mostly robots and volts kill electronics so thats why its more effecting then caps

I've managed to pick one up from a CELL operative quite early in the game, they're not very common I admit, but once you pick on up, every ammo caché stocks the batteries needed to fire it

Though you will lose it mid-way through the campaign when you lose all your weapons, then you have to wait until you encounter more CELL troops.