The ending of a noun changes depending on whether it is singular or plural and on its case (the grammatical role it plays in the sentence). We must learn the "ending patterns" for each type of noun -- masculine animate, masculine inanimate, feminine, and neuter. See my comments lower on the page regarding information available in the Tips and Notes.
This is not a very good place to discuss the details, especially considering you already checked some videos. Search the internet for written descriptions, especially those that concern Duolingo. It was discussed here before and at better places than this. See threads like https://www.reddit.com/r/czech/comments/a76elh/how_do_you_pronounce_%C5%99_as_a_native_english_speaker/ If descriptions like that do not help you, use the descriptions of the required position of the tongue, use linguistically oriented descriptions like https://en.wikipedia.org/wiki/Dental,_alveolar_and_postalveolar_trills#Voiced_alveolar_fricative_trill https://simple.wikipedia.org/wiki/Alveolar_trill#Raised_alveolar_non-sonorant_trill https://en.wikipedia.org/wiki/Voiceless_alveolar_trill#Voiceless_alveolar_raised_non-sonorant_trill