I oftes see videos where people describe (what I believe is hop-related aroma) with words like spicy, floral, citrusy, etc., but for me it is very hard to get (and to tell apart) any of this. So what are the main aromas that describes hops, and most important, how can I train my nose to recognize at least the main hop-realted aromas?
For some hands on learning with less effort required than brewing several SMASH batches, you can dry hop some bland beer as explained here. I did not write that nor have I tried it yet but it looks like an interesting experiment.
A great way to learn the distinct flavors of hops/grains is SMaSH (Single Malt and Single hop) brewing.
By using a single grain and a single hop you can really focus on one flavor at a time. So if you make a beer that is 100% cascade, you can expect to taste a flowery and spicy, grapefruit flavor. Once you get a feel for what that tastes like move on to a new hop. Using this method over several varieties of hops can really train you to look for a distinct aromas and flavors.
The only way to really get a good feel for flavors is experience.