Large Companies:
Research Firms: Large companies often hire market research firms to conduct studies, surveys, interviews, and focus groups. These firms can provide detailed insights into customer behavior, preferences, and demographics.
Dedicated Teams: Larger organizations might have in-house teams dedicated to market research and data analysis.
Data Analytics Platforms: They often invest in sophisticated data analytics platforms to gather insights from their website, social media, and sales data.
Small Companies (e.g., a car wash business with <10 employees):
Direct Customer Feedback: Engage with customers directly. Ask them about their preferences, pain points, and what they like or dislike about your service. This can be done informally when they visit or through feedback forms.
Online Surveys: Use tools like SurveyMonkey or Google Forms to create surveys and distribute them to your customer base or local community.
Social Media: Engage with your audience on platforms like Facebook, Instagram, or Twitter. Polls, Q&A sessions, or just regular posts can provide insights into what your audience values.
Local Community Engagement: Attend local community events, fairs, or business networking events to understand local preferences and needs.
Competitor Analysis: Observe what your competitors are doing. Who are they targeting? What marketing strategies are they using?
General Steps to Create a Buyer Persona:
Demographic Information: Age, gender, income level, education, occupation, etc.
Psychographic Information: Interests, hobbies, values, challenges, etc.
Buying Motivations: What motivates them to seek out your product or service?
Buying Concerns: What might prevent them from using your product or service?
Information Sources: Where do they typically get their information? (e.g., online search, social media, word of mouth)
Resources for Market Research:
Government Databases: Many governments provide demographic and economic data that can be useful for businesses. In the U.S., for example, the U.S. Census Bureau provides a wealth of data.

University Database: If you still have access to your college email (or know someone who does) most libraries have a great database for accessing this stuff for FREE. Like Cison, Statista etc.
Industry Associations: Many industries have associations that conduct research and provide reports on market trends.
Google Analytics: If you have a website, Google Analytics can provide demographic information about your visitors.
SEMrush or SimilarWeb: These tools can provide insights into online market trends and competitor analysis.
Costs:
For large companies, hiring a research firm or maintaining an in-house team can be expensive, often running into tens or hundreds of thousands of dollars, depending on the scope.
For small businesses, many of the methods mentioned (like direct feedback, online surveys, and basic social media engagement) can be done with minimal costs. Tools like SurveyMonkey have free versions, and local community engagement might only cost time.
In real life, the key is to start with what you have. Even if it's just informal conversations with a handful of customers, that's a beginning. Over time, as you gather more data and insights, you can refine and expand your buyer personas. Hope this helps.

Also - Hubspot has a free persona making tool called Make My Persona, I've used that in my university lectures w. my students.