In fact, algunos(masculine plural) / algunas(femenine plural) are indefinite so that means that your talking about apples in general. And it would be related to some.
e.g.
Some apples are red. Some apples are green.
Algunas manzanas son rojas. Algunas manzanas son verdes.
But unos(masculine plural)/ unas(femenine plural) is more specific to the apples you are talking about. In this case you have to use some of
e.g.
Some of these 50 apples are red.
Algunas manzanas de éstas 50, son rojas.
But careful, in case of uno/una (singular) is also used as the indefinite article a/an in English .
e.g.
There is an apple in my bag.
Hay una manzana en mi bolsa.
I want to eat an apple right now!
¡Quiero comer una manzana en este momento!
But careful again, uno/una is also in Spanish an expression of quantity. Used to count things and in this case is equal to one.
One apple, two apples, three apples.....
Una manzana, dos manzanas, tres manzanas....
And finally, Apples in spanish are related to the femenine sense, so you must use una/unas/algunas
e.g.
**Correct**
Una manzana, unas manzanas, algunas manzanas.
**Incorrect**
Uno manzana, unos manzans, algunos manzanas.
UDPATE
Alguno/alguna (singular) is still indefinite but in contrast to ninguno (none) as you found on Internet. It depends on the context because alguno/alguna is used as an indefinite pronoun also. In this case you should use someone/somebody/anyone
e.g.
Imagine that you are talking to someone else and you are about to go hiking in a group. And you want to know if someone came...
Someone came?
¿Alguno ha venido? --> Careful, this is more common in Spain.
¿Alguien vino? --> This has the same meaning. Used in Spain but it's more common in Latin America also. It's more natural for Latin Americans to say like this.
alguien is an indefinite pronoun. Used exclusively for people.
You can use alguno/alguna for your apples but it's still indefinite (but not as a pronoun). But in this case you wil use any commonly for questions.
Is there any apple on the table?
Hay alguna manzana sobre la mesa?
This is in order to know if there is at least one!
