Roman colonia were settlements with Roman inhabitants in non-Roman territory. These were often carefully planned and the land was divided up and often given to retired soldiers. The soldiers were accompanied by officials who measured the land and made the decisions about who got what. These settlements were led by duoviri or quattuoviri while the local senatus functioned as a city council. Members of this council were called decuriones. It was basically the organization of the government as a whole, but in a smaller format.
Within these settlements often lived local elites, or sometimes even provincial elites. To make a name for themselves these elites would gift buildings to the settlement. The benefit was two-sided. The settlement would gain new buildings that would enhance their quality of life, and the local elites who made the gift would increase in standing. The elites who made these gifts had a lot of influence in the architecture, which would always be designed in such a way to provide them with maximum exposure for their family's benevolence. Of course, it was useless or frowned upon to gift buildings a city did not need, so this was rarely the case. To maximize their own standing they would look carefully at what was lacking and choose accordingly. This way all the needs of the town were met, at the expense of the local elites, benefiting all parties involved (city, elites, townsfolk).
Interestingly, roads had a bit of a different dynamic to them. The main roads like the Via Appia were created by the emperor himself. They were likely created to improve military response times, but also helped in improving travel between various colonies. It was the responsibility of cities to connect to the larger main roads, and the cities themselves were responsible for the maintenance and costs of these connection roads. These were mostly funded by taxes. While some elites might choose to invest in building a road, it was an expensive endeavor and did not provide nearly as much self glorification as a giant new bathhouse right smack in the center of town, thus it mostly fell to the municipalities themselves to fund these mandatory connections.

So in short: colonia were founded by the state, the land was then divided up by officials and the basis of a small settlement was created. In already existing cities, or colonia that were large enough local elites would move in and start gifting buildings to improve the standing of the city, and thus increase their own reputation. This was then organized by the city officials, while influenced by the gift-giver. Roads on the other hand were mostly done by the government itself. Main roads were paid for and maintained by the emperor, while connecting roads were paid for and maintained by local municipalities.

Yntema, Rome en het Romeinse Rijk, 2014. Laurence, The Roads of Roman Italy: Mobility and Cultural Change, 2002. Radke, "Viae Publicae" in Der kleine Pauly: Lexicon der Antike, 1964-1975.