A lot of words but not many sources here. So here's a few:
Marx defined socialism as: "...Socialized man, the associated producers, regulate their interchange with nature rationally, bring it under their common control, instead of being ruled by it as by some blind power; they accomplish their task with the least expenditure of energy and under conditions most adequate to their human nature and most worthy of it. But it always remains a realm of necessity. Beyond it begins that development of human power, which is its own end, the true realm of freedom, which, however, can flourish only upon that realm of necessity as its basis."
— Capital III, translated by Ernest Untermann, Charles H. Kerr & Co., Chicago 1909, p. 954
To understand Marx's definitions you have to realise he was writing mostly in response to the Paris Commune uprising and therefore saw 'communism' as the practical application of a theory of socialism. However, the terms and their meaning were radically reshaped by Lenin, Mao and Stalin.
— The Paris Commune: First Proletarian Dictatorship, Revolution, Vol. 3, No. 6, March 1978.
In March 1918 the Bolshevik Party was renamed the Russian Communist Party (Bolshevik) in order to distinguish it from Social Democratic parties in Russia and Europe and to separate the followers of Lenin from those affiliated with the nonrevolutionary Socialist International.
— https://www.britannica.com/place/Soviet-Union/Lenin-and-the-Bolsheviks
Sure, but Marx didn't invent socialism. The Commune itself was massively inspired by Proudhon's socialist ideas. Even before that, Saint-Simon's socialism influenced some factions that took part in the French Revolution.
All that to say that yeah, nowadays Marxism is the main socialist ideology, but it's not the only one