Okay, I'll freely admit that I'm a nobby newby cyclist and at the minute I just have zero comprehension of most of the terminology and options for my bike. I bought a Genesis (CDF 20) gravel bike in February and love it to bits but there's still so much I don't understand:

1. I simply want to buy a spare inner tube and, despite trying to read up on it, I still have no idea which one to get. From my bike's spec I know the wheels are apparently size 700c and that the tyres are Kenda Booster 700 x 40c's. I'm guessing the 40c is something to do with the width/height of the tyre? So daft question, what size inner tube do I need? If I search on-line I get all sorts of size inner tubes thrown at me, showing a range of size which may or may not be relevant, and I just simply want to know what to get. (Yeah I appreciate I could also go into my local cycle shop and ask )

2. A direct quote from the Genesis web site: 'The ability to run up to a 45mm rear tyre and a 50mm front tyre on the provided 700c rims increase the all-terrain ability of the CDF 20. If you want to experiment with 650b wheels, you could run up to a 44mm rear tyre and 50mm front tyre'

Assuming the 40c tyres that I currently have means that I'm running 40mm tyres, I'm guessing that means that I can get bigger tyres? And presumably bigger front than back if I want to? And why would I want to? Extra grip, durability and/or off road ability I assume? And presumably 650b wheels are smaller circumference wheels that offer more off-road something or other? More mountain bikey performance from the wheels I assume?

3. The absolute best gobbledegook is from a third-party review of my bike on their web site. Talking about the GRX chainset, which might be a 46/30 tooth paired with an 11-34t cassette (which I'm guessing is the range of gear cogs) it goes on to "helpfully" elaborate as follows:

'If you want something even lower, then the GRX 400 rear mech will accept a 36-tooth sprocket which can be achieved by using a CS-HG50-10 mountain bike cassette from Shimano'. I'm not sure I understood a single word in that sentence

Can somebody patiently explain to me wtf this all means? Thanks