If the modern armchairs and sofas belong to the same collections, the problem of matching doesn't arise. The alternative is to choose living room armchairs and sofas from different collections, but within the same same style, for a more dynamic and personalized ensemble.
If desired, the shades can also be different, as long as they match well. The burgundy model, for example, can be paired with a dove gray sofa, but not with a sofa with light blue upholstery.
Alternatively, the common thread between sofa and armchairs can be color, and you can take greater stylistic freedom, but it's best to avoid excesses, that is, those differences that threaten to appear abysmal. Another recommendation for everyone: be careful to respect the proportions.
There are different types of armchairs, it's true, but it's undoubtedly useful to list the main ones:
Chesterfield: the classic leather armchair with buttons, a low seat, and armrests and a backrest at the same height. Bergére: its unmistakable feature is the high backrest with "ears."
As for modern models, it should first be noted that very often tradition is the main inspirational muse, the starting point for ad hoc reinterpretations. For those who don't have much space, there are easily placed armchair models, capable of personalising and characterising the environment with their design or the quality materials from which they are made. You'll find everything in the product data sheet.