Quick Russian question

So I am finally trying to do a bit of work on learning Russian grammar. I am so far quite confused and I have a quick question. I am looking at the noun declension tables on this useful grammar website

The Prepostional Case of Russian Nouns ← for the prepositional case
Genitive Case of Russian Nouns ← for the genitive case

For each case, the tables give instructions for how to decline nouns in that particular case given their gender and ending. I notice though, that none of the tables on this website say what to do for feminine nouns ending in -я. Does anybody know why they don’t say anything about this? Is it that the declension works in the same way as the feminine nouns ending in -ь?

Not always.
Compare Genetive and Preposional of different feminin nouns:
сестрА- у сестрЫ - о сестрЕ
деревня - у деревнИ - о деревнЕ
армИЯ- у армИИ- об армИИ
мышь- у мышИ - о мышИ
Of course, you can everything find in the Internet, but in vain you don’t use enough my grammar lessons here in Lingq.com because I try to fasilitate all grammar rules and pay attention to the most important points.
For example you can find opening the lessons from 91 to 98 of my collection ПЕРВЫЕ ШАГИ(“FIRST STEPS”) where I explain 3 genders of Russian nouns and then give the declension of every gender separately.