Magic: The Gathering

Stack still exists and I think those rules are mostly unchanged from 2013.

I haven't played tournaments in a while, but instead of the DCI I believe they are using the Wizards Account. If you have played Magic Arena then you already have one, if you haven't it might be worth installing and playing a bit to get a refresher on the game and see some of the recent sets.

There's a store locator that may help find some place to play: https://locator.wizards.com/

Finally, about building decks... first find out where and against who you're going to play. Since there's a bunch of different formats (Legacy, Modern, Pioneer, Standard, Commander) it's best to find out what is popular at the place you want to play before building a deck.

The stack (and priority) is still there? Not sure where you got that. https://mtg.fandom.com/wiki/Stack

Everything about changes to DCI is here: https://mtg.fandom.com/wiki/DCI

Not sure if there is a map of (registered/official) clubs somewhere but https://www.google.com/search?q=mtg+scotland gives a lot of hits.

About deck ideas there is:

• https://www.mtggoldfish.com/
• https://aetherhub.com/
• https://www.mtggoldfish.com/metagame/standard#paper

Important thing will be to decide what format(s) you will jump into, I play commander with a friend but the local club (and basically whole city) only plays modern (https://magic.wizards.com/en/formats/modern, more about current formats here https://magic.wizards.com/en/formats).
Personally I wouldn't recommend jumping into standard as it will require the most money to get a functioning deck and to keep that deck actual - https://mtgazone.com/standard-rotation/

If you're in Edinburgh you can go to Mox in the Hole. Commander is the leading format by a large margin nowadays, so the best place to start is either going to be a precon commander deck or picking out a favorite commander of yours to build around.

The stack is still there.

Mtggoldfish has meta various decks on there.

Most people now play the commander/EDH format. There’s also now Pioneer in addition to modern.

For sets the power creep really started ramping most noticeably in Throne of Eldraine.

Additionally they sell different types of packs now. There are draft booster (classic type you’re familiar with, set boosters (more focused on people trying to collect the set) and collector booster (much higher priced and contain various alternate art cards)

The Stack is still there and works like it did.

I have no idear how competative magic is organiced since they changed it so often.

What to play is dependend on what you want to play, but mtggoldfish is a good start to have an inside look at the current meta.