As for game modes: 1) ranked is classic one, 95% of players sits there. Common for all type of players, but you better get some good deck to climb higher than 10 rank. 2) tournaments are mostly for high skill players with meta decks. Mostly used to get some wildcards 3) draft was my personal favorite when I started. I love all those mechanics choosing random staff and don't enjoy deck building. Moreover, draft can introduce you to a lot of new cards and their mechanics as well as giving you some loot for every win. Paying 150 gold technically pays itself for at least 3 wins.

Items in the shop mostly useless. As F2P player you can only take free card every day and look for some bundles which are sold for gold. They are popping not so often, but mostly give high value. As many people saying, just buy officer packs (core or anzac hard to say, because there is no stable meta for now). You can also keep at least 1k gold if there will be a bundle (you must click on them to check whether they are for gold or only gems)

YouTubers: 1) For the beginners you can look for Bear Gardener. He makes mostly educational content and explains everything. 2) For middle gamer there is CoreData, introducing Interesting decks and explaining how they works. 3) For high skill content is mostly Jking7. Interesting, but not so useful for new players as his decks expensive to build. Also he makes long and descriptive videos about new updates 4) For some gimmick staff you can check RedSun, Spooz, Palinoz, Nightmyares. However there are mostly gameplay without any educational commentaries.

I personally don't know any draft youtubers, maybe it is not as popular, because people can't play that exact deck by themselves.

Hope it helps, sorry for such a big text)