The Brief Young Adult Alcohol Consequences Questionnaire is used to assess the severity of alcohol consumption. Higher scores indicate greater severity of consequences associated with alcohol consumption.

The Brief Marijuana Consequences Questionnaire is used to assess the severity of marijuana consumption. Higher scores indicate greater severity of consequences associated with marijuana consumption.

Brief Symptom Inventory is used to measure depression, anxiety and somatization symptoms to assess for psychiatry distress. Index ranging from 0 to 72 with cut-score of 11 as indication of psychiatric distress.

Youth's living situation questionnaire will be used to gather information related to the youth's living situation over the past 4 months (e.g., in the community, residential treatment, or other living situation).

We will administer an 18-item yes/no questionnaire to gather information on what youth perceive as most relevant and important social determinants of health items for foster youth (i.e., employment, food, family, healthcare, housing, legal, education, safety at school/work, transportation, "other" needs).

The Working Alliance Inventory (WAI) is used to assess youth's perception of therapeutic alliance (with provider) over time. Scores range from 12-84 with higher scores indicating greater perceived therapeutic alliance.

Frequency and quality of CASA-youth conversations about substance use: For frequency, we will adapt an item from a validated measure of parent-child communication about sex used in previous studies45 "How many times have you and your CASA talked about [alcohol, marijuana, cocaine]" (each drug asked separately). For quality, we will use one-item from the scale that assesses perceived helpfulness 'How helpful do you think your discussions about alcohol and/or drugs were?' and one-item that assesses perceived comfort 'How comfortable are you with these discussions about alcohol and/or drugs with your [CASA youth] or [CASA].

6: deidentified assessment of Adverse Childhood Experiences (10 items) and Related Life Events (7 items).

The Scale of Ethnic Experience is used to measures youth ethnicity-related cognitive constructs (e.g., perceptions of their own ethnic group, ethnic group belonging) in four domains: Ethnic Identity(12-items; α =.87); Perceived Discrimination (9-items; α = .91); Mainstream Comfort (6-items; α=.87); and Social Affiliation (5-items; α =.83). Items rated on a scale from 1 (strongly disagree) to 5(strongly agree).

The Social Distance Scale (SDS) is used to assess a person's willingness to interact with others with mental illness (SDS) or substance users (SDS-SU).

The Monitoring the Future is used to assess how many close friends are using alcohol, cannabis and other drugs occasionally and regularly.

A Demographics Questionnaire is used to gather standard demographics (e.g., age, gender, race) and information about employment, number of years as CASA volunteer, length of time known current foster youth, immigration status, country of origin.

Beliefs about youth SU will be measured using an adapted 9 item perceived risk of SU measure used in studies of similar risk perceptions (among parents) associated with youth substance use. Items such as "If your youth were to use alcohol more than once a week, what do you think are the chances of them becoming (for example) addicted to alcohol, developing cancer (or lung cancer for drugs that can be smoked), experiencing brain injury etc.

The Professional Quality of Life will be used to gather information about compassion satisfaction and compassion fatigue.

Number of app log-in's, days of use, time spent using app, number and type of features used e.g.,chat use (peer/FAB), chat use (navigator), number of clicks on SU prevention content (e.g., SU resources requested, peer "adulting 101" SU prevention video watched), requested navigator support (yes/no), requested navigator support about SU (yes/no), requested teletherapy [primary presenting need, type of intervention, e.g., primary SU or MH or co-occurring, modality of intervention (individual, group, family), number and frequency of sessions received].

The Emotional Wellness Questionnaire is used to assess psychiatric symptoms (e.g., depression, anxiety, mania) and substance use.

The System Usability Scale (SUS) is used to gather information about the overall usability of web applications.

10- item measure widely used for web applications and proven superior to other measures. SUS yields a single number representing a composite measures of the overall usability of the system being studied60,61. Scores will be compared to established norms for levels of usability. Our goal is a score of 80 (top 10% of scores), which reflects excellent usability, and is associated with recommending the product to a friend.