Worry not. It isn't dangerous. If the pressure goes above 3 bar - about where the red bit is if there are no numbers, then water will push open the aforementioned prv, which is sprung, and water will go outside. It might well dribble afterwards but no danger and not hugely expensive to replace.
If you did turn the boiler on, water would heat and expand and put the pressure up more, so if you can wait, do.
They make them so they don't explode, whatever someone does, or goes wrong (within reason!). That would be lots of paperwork and terribly bad for business.
The blue circled things are drain cocks as described above, on which you turn the square head. Either will do. Garden hose will be too big to stay on the outlet but you could hold a bit to discharge into a sink/w-up bowl. I see harder alternatives but won't mention them ( ).
The pink outlined silver braided hose is the "filling loop" which should be disconnected one end, so water can't continue flowing into the boiler. Looks like it is.
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