The easiest way is to sweeten with a non-fermentable sweetener, however many (including me) find that the taste isn't to their liking (some produce gastric effects or in the case of Xylitol can be toxic to dogs). You will no doubt get lots of advice on this.
If using sugar, you need to add it to the finished cider then heat (or chemical) pasteurise to kill the yeast and stop the sugar from fermenting. Look up my post of 1 Feb 2021. It goes into a lot of detail regarding heat pasteurising for carbonation and sweetness. Others know more about chemical pasteurising than I do.
A few guidelines are...
Add sugar to a sample of your cider to see what level of sweetness you want, then see what SG this relates to. Add sugar (easiest as sugar syrup), AJ etc to bring the whole batch up to this SG, then bottle. Others may suggest adding the sweetener to each bottle but I find this is a bit messy and not consistent from bottle to bottle.
I like just a touch of sweetness (SG 1.004 or about 10g/L of sugar). This is something like1/2 teaspoon of sugar in a cup of coffee. As a guide, the BJCP (Beer Judge Certification Program) has 5 categories of sweetness by residual sugar... Dry <0.4% (SG<1.002), Med-Dry 0.4-0.9% (SG 1.002-1.004), Medium 0.9-2% (SG1.004-1.009), Med-Sweet 2-4% (SG1.009-1.019, Sweet>4% (SG>1.019).
If you are looking for sweetness and carbonation, then you need to add extra sugar (i.e. increase the SG) for the carbonation then pasteurise once the carbonation is achieved. Roughly, fermenting one SG point will result in 0.5 volumes of CO2. Most cider is carbonated to 2.0-2.5 volumes.
Bottle pressure ratings are hard to get. Ordinary new12oz (330ml) bottles are rated by China Misa Glass at 1.2-1.8MPa (174-261psi), whereas Visy recommend filling with not more than 4GV (gas volumes).
Grolsch (flip top) type bottles can leak above 70psi of pressure so have a built-in safety feature (but don't rely on it).
I prefer waterbath heat to chemical pasteurising and work on 150psi as being my upper limit when doing this. Generally I limit this to not much more than 100psi, but others may have different views. FYI, a bottle carbonated to 2.5 volumes will reach 109psi at a pasteurising temperature of 65C. In a "worst case" if you forget to pasteurise, a medium sweet cider (at SG1.016) can generate 8 volumes of CO2 and reach 150psi pressure at warm room temperatures.
Be careful, remember your PPE and have fun!