The e in arquiteto is an accented e. It is an open e, pronounced 'eh' with the mouth open. T and d do not change when followed by an open e. Portuguese also has a closed 'e', pronounced somewhat like 'ay' (actually more like 'eh' but with the mouth half closed). It also has an unaccented 'e' pronounced more like the i in 'it'. T goes to ch and d goes to j when followed by an unaccented e
In Portuguese, when talking about occupations and job positions, they themselves become an adjective to the person with the verb ser. You can use the indefinite article but it's far less common.
The definite article, in this and many other contexts, we use to specify a person. Basically the same usage as in English.
Você é enfermeira? — Are you a nurse? [COMMON]
Você é uma enfermeira? — Are you a nurse? *(the same meaning in this context) [LESS COMMON]
Você é a enfermeira? — Are you the nurse? (specifying the nurse, it has a different meaning — you may guess what it is)