Study to Measure Filgotinib in the Blood of Children and Teenagers With Arthritis Taking Filgotinib (SCALESIA)

Key Inclusion Criteria: * Participant with a body mass index (BMI) within the 5th to 95th percentiles for the age and gender (based on World Health Organization BMI charts). Participant must have a minimum weight of 15 kg. * Participant must meet the International League of Associations for Rheumatology classification for 1 of the following categories and have, according to the investigator's judgment, moderately to severely active disease that is not adequately controlled with his/her current therapy. * Rheumatoid factor (RF)-positive polyarthritis * RF-negative polyarthritis * Oligoarthritis * Psoriatic arthritis * Enthesis-related arthritis (ERA) Note: Historical Human leukocyte antigen B-27 (HLA-B27) results are considered appropriate for ERA diagnosis during screening. * Systemic JIA with active arthritis without active systemic features, or with active systemic features that are stable in the prior 6 months of time of enrollment * Participant with intolerance or a history of inadequate response to at least one of the following medications for the treatment of JIA, administered for at least 12 weeks, based on current treatment guidelines: conventional synthetic disease-modifying antirheumatic drugs and biological disease-modifying antirheumatic drugs (including methotrexate) and non-steroidal anti-inflammatory drugs for ERA and psoriatic arthritis. * Female participants of childbearing potential (i.e. who have passed menarche) must have a negative highly sensitive urine pregnancy test. Key Exclusion Criteria: * Participant with persistent oligoarthritis. * Participant with undifferentiated arthritis. * Participant with any other any other rheumatic, inflammatory, or immunologic disease (e.g. inflammatory bowel disease, hypogammaglobulinemia, systemic lupus erythematosus, or uncontrolled uveitis). * Active infection that is clinically significant, as per judgment of the investigator. * Participant with a history of complicated herpes zoster infection (with multi-dermatomal, disseminated, ophthalmic, or central nervous system involvement). * Currently on any therapy for chronic infection (such as pneumocystis, cytomegalovirus, herpes simplex, or atypical mycobacteria). Note: Other protocol defined Inclusion/ Exclusion criteria may apply.