Ride your bike athletically for long distances on the roads and you are a road cyclist.
There's nothing more than that. As long as you are happy with the bike there's no secret sauce.
I'd say 99% of those who ride bike athletically for long distances on the roads find that bike with no suspension suits them best, that drop handlebars suit them best (and that it's a good idea to ride on a handlebar covered by tape instead of a bare handlebar), that relatively narrow (32mm or less) tires with high TPI, no tread pattern (slick) and very thin sidewalls suit them best, and that some kind of clipless pedals suit them best. Also 99% of those who ride on the road choose to ride on usually synthetic fiber clothing and perhaps wool as an alternative and avoid cotton clothing. 99% of the road cyclists either have a double crankset with two relatively large chainrings or alternatively don't use their smallest ring on the triple crankset at all and have decided if they need to purchase a new bike it'll have a double crankset.
Most road cyclists choose to ride on bikes that don't provide any electric assist at all, but for those that are very heavy (over 100 kg) and have avoided sports for a long time and wish to restart sports by doing road cycling, it's not a crime to use an electric road bike, should you find one (finding one could be a bit tricky though).
Don't be afraid to call yourself "road cyclist" even if you use for example SPD pedals that are officially considered "MTB" technology. With SPD pedals, you can ride 50 km on the road, stop at a cafe without walking like a duck, ride another 50 km on the road and not having to place an order for new plastic cleats that were damaged because you walked like a duck on them instead of riding on the bike non-stop.
A gravel bike, or a drop bar touring bike, or a cyclocross bike doesn't make you less of a road cyclist than a carbon fiber
6799.999999 gram 6800.000001 gram carbon fiber road bike would make you.
If you are short on storage space and/or money, your road bike can be the same as your commute bike. This means you can have pannier rack, kickstand, mudguards, mounted U lock, bell, lights, reflectors, hub dynamo and other essentials on your road bike.
There are even road cyclists who dare to ride without a water bottle! (Of course having the dreaded SPD pedals will make it slightly easier to ride without a water bottle as you need the ability to stop at a grocery store to buy something to drink without walking like a duck if you choose to ride without a water bottle.)
Also if your leg strength is excellent but your hand strength is your weak point, you can perfectly well use a bike where the handlebar is only a couple of centimeters below the saddle or so, you don't have to ride a bike with excessive saddle-to-handlebar drop to be a road cyclist.
Is there any comprehensive and standard beginner-guide to the road-cycling?
You don't become a road cyclist by reading a guide.
You become a road cyclist by riding your bike athletically on the road for long distances.
However, it may be useful to at least be aware of drop handlebars, high performance slick tires, clipless pedals and non-cotton sports clothing. Those are the things that for 99% of road cyclists make their road cycling far more enjoyable. I don't think there are very many road cyclists that try some of the items on the list and decide they don't need that item.
Also you probably discover on your own the various riding positions on a road bike but in case you don't, here's the list:
- When riding seated, you can hold your hands on the tops, on the corners, on the hoods and on the drops. Positions on the left are less aerodynamic but cause less fatigue of your arms. Positions on the right are more aerodynamic but cause more fatigue of your arms.
- When riding up hills, if you choose to go faster than you can comfortably go seated it's easiest to ride standing, hands on the hoods (climbing position)
- When needing very large amounts of power for short bursts, if you choose to do so, it's easiest to ride standing with hands on the drops (sprinting position)