This is another long winded answer but there are some things you need to know before trying to go down the sweet and carbonated path.
After fermentation, the cider will continue to mature and develop flavour for quite a while so just letting it sit for an extended time can change things for the better. I guess your Somersby is similar to ours here in Oz... carbonated with some sweetness. Commercially this is usually achieved by adding a sweetener and force carbonation.
For us amateurs, "sweet carbonated" is sometimes seen as the holy grail since this is what most people buy. It can be achieved but requires a bit of juggling of several different options. Here are some choices but you will have to do some research (here and in other places) to decide which way you want to go.
Sweetening... the easiest is to add a non fermentable sweetener (not sugar) such as Xylitol, Splenda, etc. More difficult but "traditional" is to stop fermentation above1.000. Generally most cider is somewhat sweet from 1.010 down to 1.005, after that the sweetness declines quite quickly. You can use heat or chemical pasteurisation to stop fermentation at whatever sweetness you want. I have found that some yeasts will retain a certain amount of sweetness/flavour. Currently WLP775 is my favourite, with SO4 often stopping at 1.002-1.004, thus leaving a bit of residual sweetness. 71b is also reputed to be good for this but I haven't tried it.
Carbonation... for bottle carbonation, you need to bottle at several gravity points above where fermentation finishes (typically 1.000) although as above, I have found that SO4 can stop above this. Typically a 0.001 drop in SG will generate 0.5 atmospheres of CO2. So for a typical 2 atmosphere carbonisation you will need to bottle 0.004 gravity points above where you think fermentation will finish.
So, it is fairly easy to get non-sweet carbonated cider or sweet still cider, but getting both is a bit of a challenge without resorting to forced carbonation... but it can be done.
One approach (which I haven't tried) is to ferment completely then add apple juice concentrate up to the gravity you want for carbonation (say 1.004). Bottle this and rely on the AJC to add apple flavour and sweetness. Others who have done it may like to chime in on this.
A more involved (and "pure") approach involves bottling at say 0.004 above the sweetness you want (say you want 1.006 sweetness, so you bottle at 1.010). Then, you heat pasteurise when it has fermented down to 1.006 or generated a bottle pressure of 2 atm (in theory, the same thing). How do you know what is going on inside the bottle? One way is to monitor the rate of pasteurisation (i.e. drop in SG per hour, day or whatever) then extrapolate this to estimate when the desired SG is reached. Another is to have a sealed sample/test plastic bottle of the cider and give it the squeeze test. Around 2-3 atmospheres of carbonation is where most soft drink bottles are at... i.e firm when you squeeze them. So you then heat pasteurise at that point to stop any further carbonation and end up with sweet carbonated cider.
Pasteurisation by heat starts at about 60C, but 65C (149F) for 10 minutes is considered to be a good rule of thumb.
Getting this right can be fraught with peril as heating carbonated bottles can take you into bottle bomb territory (it has happened to me!!!). The sticky at the top of the page and subsequent posts goes into this in some detail. Bembel's post of 11 May 2016 gives a good low temperature method of how to reduce the chances of bottle bombs. Similarly, do a forum search on Cooler Pasteurisation.
Some things to consider when using bottles... ordinary light weight beer bottles are generally considered O.K. for up to 5 or 6 atmospheres but this is pretty much the safe limit. Doubling the temperature will more or less double the pressure inside the bottle, so heating for pasteurisation to more than double the room temperature (which is needed) can lead to kaboom unless you are using particularly robust bottles (like champagne bottles).
Phew... good luck!
