Esta harina blanca es la que uso siempre para la masa.

Esta is a demonstrative adjective meaning this, used when you’re pointing out a specific item (physically or in the conversation): esta harina = this flour.

• La harina would mean the flour (specific, but not necessarily “this one here”).
• Una harina would mean a flour / some flour (more general, often implying a type of flour).

In Spanish, descriptive adjectives like blanca usually come after the noun: harina blanca.
Putting the adjective before the noun (blanca harina) is possible but sounds poetic/literary or changes emphasis (more subjective or stylistic). In everyday speech, harina blanca is the normal choice.

Es la que... is a very common structure meaning (it) is the one that.... It’s used to identify something and then add a relative clause:

• Esta harina blanca es la que uso siempre... = This white flour is the one (that) I always use...
  Here, la que refers back to harina and introduces the clause uso siempre....

You can sometimes say Esta harina blanca es la que uso... or Esta harina blanca es la que... very naturally, but in this specific “the one that…” identification structure, Spanish typically uses the article + relative pronoun: el/la/los/las que.
Saying Esta harina blanca es que uso siempre... would be incorrect because es que is a different structure meaning something like it’s that... (often giving an explanation), not the one that.

Because it refers to harina, which is feminine: la harina. The article matches the noun:

• la que (feminine singular) → refers to la harina
• el que (masculine singular) would refer to a masculine noun
• lo que is used for what / an abstract idea, not for a specific feminine noun like harina.

Spanish often drops subject pronouns because the verb ending already shows the subject. Uso clearly means I use.
You can say yo uso, but it usually adds emphasis or contrast (e.g., Yo uso esta, pero tú usas otra).

It’s flexible, but some positions sound more natural than others. In your sentence, uso siempre is very natural. Other common options:

• ...la que siempre uso para la masa. (also very common)
• ...la que uso para la masa siempre. (possible, but often less natural unless emphasizing always)

Para expresses purpose or intended use: for (what it’s used for).

• para la masa = for the dough / for making dough

Por is used for different ideas (reason, exchange, duration, movement through, etc.), so por la masa would not express the intended meaning here.

In cooking contexts, la masa commonly means dough (bread dough, pizza dough, pastry dough, etc.).
It can also mean mass in other contexts, but with harina (flour) the cooking meaning is the obvious one.

Yes, that’s also correct and very natural. It just reorganizes the sentence:

• Esta harina blanca es la que uso siempre para la masa. (focuses on identifying this flour as “the one I use”)
• Esta es la harina blanca que uso siempre para la masa. (starts with this (one) and then specifies the white flour)

Both are normal; the choice is mostly about style and what you want to highlight first.