I have both a Playstation 4 and a Playstation 5.
Do you have Nintendo Switch? 
I do not. I haven't had many Nintendo console systems.
I think Nintendo games are extremely cute and interesting! I don't play a lot of PS :(
Is it free to play the Nintendo Switch online?
In order to play Nintendo Games you gotta have the console itself. It's worth it. It has so many awesome games of any genre
Are there any Mario or Super Smash Bros games available on the Switch?
yes! Many Mario universe games: Yoshi's crafted world, toad treasure tracker, mario kart, mario odyssey, mario party etc. and Super Smash bros as well! 
That is awesome! I used to play Mario Games all the time on Gameboy.
Same here! You can also play old Mario (and not only mario games) on Switch for free