In general use : Your understanding is correct.

Take care with some nouns that are, in particular, strictly countable, but can convey some uncountable general meaning :

With currencies for instance :

• Currencies are strictly countable, and, in a bank, tu échanges des euros contre des dollars
• However, when used with the general meaning of money (which is uncountable) tu achètes de l’euro et vends du dollar

“Vin” and “pain” are uncountable (“nom massifs”) as in English and that means that you can’t normally use the indefinite article or numbers to determine them and instead that you can use partitives (de, un peu de, etc.). So, “du pain” and “du vin” is correct.
However “pomme” is countable and so that’s the opposite that is true: you can use the indefinite article and Numbers (une pommes, des pommes, trois pommes,…) but in no way partitives; you can’t say “de la pomme”.

You should say this:

“acheteter des pommes”.

You can’t say “un verre du vin” unless you add a complement or an adjective. The function of those addition is to determine the noun so that a definite article can be used (le, la, les).

• boire un verre du vin qui est sur la table (de le vin)
• boire un verre du vin vieux et pas du nouveau
• boire un verre du vin frais (this means “a glass from that wine that is cool and not a glass from another wine that is also there but that hasn’t been refrigerated. “Du”, here, stands for “de le”.)

You can also say “boire un verre de vin frais” and in that case there is no caracterisation of the wine with respect to another wine but a simple caracterisation in itself; in that way there is no indication that you should be thinking about another kind of wine.

Otherwise, that is without the presence of a determiner, you say “un verre de vin”, because you do not speak about a specific “occurrence” of wine but about any
wine; that’s why you use the partitive “de”.

and if I want to use le I should say acheter du vin rouge or une bouteille du vin rouge?

“Acheter du vin rouge” is correct and “du” is a form of the partitive “de” (contraction de “de le”).

If you use a construction in which “vin” is a complement of a noun, the noun being that of a container you can use “de” meaning “filled with” and “de” is not the partitive here but a preposition.

• acheter une bouteille de vin

If, however you want to specify that wine further and therefore add some words to achieve that there are three possibilities.

1/

• Elle achètera une bouteille de vin.
• Il va acheter une bouteille de vin d’Alsace.
• Ils achèteront une bouteille de vin à 10 euros.
  (In those sentences you use “de” because there is no need to mention a referent: the category is self-determined.)

2/

• Il va acheter une bouteille du vin dont il parlait hier.
  (In the present sentence, there is a referent (after the determiner “le”, which is in “du” since “du” stands for “de le”) and this referent is “that wine he was talking about yesterday”; therefore you need to add “le”, a simple “de” is not sufficient for referring to that particular wine.)

3/

— Il y a deux sortes de vin dans nos prix, du rouge et du blanc.
— Lequel tu préfères ?
— Je n’en préfère aucun mais nos amis aiment le rouge.
— Alors je vais acheter une bouteille du vin rouge.(de le, preposition)
(In this case the referent is in what precedes, in the dialogue (deux sortes, le vin rouge et le vin blanc).)

autre cas pour le féminin

— Voilà une fleur et une feuille et vous devez choisir l’une des deux comme modèle.
— Laquelle choisis-tu Jacques ?
— Je vais dessiner une image de la fleur.
— Jacques a choisi de dessiner une image de fleur.