Listing all functions available in QGIS's Virtual Layer

You can access directly to the Virtual Layers with the power of Python introspection and well formated QGIS Python modules with this code below executed in the QGIS console:

from db_manager.db_plugins.vlayers import sql_dictionary

print(f"Functions: {sql_dictionary.functions}")
print(f"Math functions: {sql_dictionary.math_functions}")
print(f"String functions: {sql_dictionary.string_functions}")
print(f"Aggregate functions: {sql_dictionary.aggregate_functions}")
print(f"Spatialite functions: {sql_dictionary.spatialite_functions}")
print(f"QGIS functions: {sql_dictionary.qgis_functions}")

Results in:

Functions: ['changes', 'coalesce', 'glob', 'ifnull', 'hex', 'last_insert_rowid', 'nullif', 'quote', 'random', 'randomblob', 'replace', 'round', 'soundex', 'total_change', 'typeof', 'zeroblob', 'date', 'datetime', 'julianday', 'strftime']
Math functions: ['Abs', 'ACos', 'ASin', 'ATan', 'Cos', 'Cot', 'Degrees', 'Exp', 'Floor', 'Log', 'Log2', 'Log10', 'Pi', 'Radians', 'Round', 'Sign', 'Sin', 'Sqrt', 'StdDev_Pop', 'StdDev_Samp', 'Tan', 'Var_Pop', 'Var_Samp']
String functions: ['Length', 'Lower', 'Upper', 'Like', 'Trim', 'LTrim', 'RTrim', 'Replace', 'Substr']
Aggregate functions: ['Max', 'Min', 'Avg', 'Count', 'Sum', 'Group_Concat', 'Total', 'Var_Pop', 'Var_Samp', 'StdDev_Pop', 'StdDev_Samp']
Spatialite functions: ['*iszipblob', '*ispdfblob', '*isgifblob', '*ispngblob', '*isjpegblob', '*isexifblob', '*isexifgpsblob', '*geomfromexifgpsblob', 'MakePoint', 'BuildMbr', '*buildcirclembr', 'ST_MinX', 'ST_MinY', 'ST_MaxX', 'ST_MaxY', 'ST_GeomFromText', '*pointfromtext', '*geomfromwkb', '*pointfromwkb', 'ST_AsText', 'ST_AsBinary', '*assvg', '*asfgf', '*geomfromfgf', 'ST_Dimension', 'ST_GeometryType', 'ST_Srid', 'ST_SetSrid', 'ST_isEmpty', 'ST_isSimple', 'ST_isValid', 'ST_Boundary', 'ST_Envelope', 'ST_X', 'ST_Y', 'ST_StartPoint', 'ST_EndPoint', 'ST_Length', 'ST_isClosed', 'ST_isRing', 'ST_Simplify', '*simplifypreservetopology', 'ST_NumPoints', 'ST_PointN', 'ST_Centroid', 'ST_PointOnSurface', 'ST_Area', 'ST_ExteriorRing', 'ST_InteriorRingN', 'ST_NumGeometries', 'ST_GeometryN', 'MbrEqual', 'MbrDisjoint', 'MbrTouches', 'MbrWithin', 'MbrOverlaps', 'MbrIntersects', 'MbrContains', 'ST_Equals', 'ST_Disjoint', 'ST_Touches', 'ST_Within', 'ST_Overlaps', 'ST_Crosses', 'ST_Intersects', 'ST_Contains', 'ST_Relate', 'ST_Distance', 'ST_Intersection', 'ST_Difference', 'ST_Union', 'ST_SymDifference', 'ST_Buffer', 'ST_ConvexHull', 'ST_Transform', '*initspatialmetadata', '*addgeometrycolumn', '*recovergeometrycolumn', '*discardgeometrycolumn', '*createspatialindex', '*creatembrcache', '*disablespatialindex', '*checkspatialmetadata', '*autofdostart', '*autofdostop', '*initfdospatialmetadata', '*addfdogeometrycolumn', '*recoverfdogeometrycolumn', '*discardfdogeometrycolumn', '*filtermbrwithin', '*filtermbrcontains', '*filtermbrintersects', '*buildmbrfilter']
QGIS functions: ['atan2', 'round', 'rand', 'randf', 'clamp', 'scale_linear', 'scale_exp', '_pi', 'to_int', 'toint', 'to_real', 'toreal', 'to_string', 'tostring', 'to_datetime', 'todatetime', 'to_date', 'todate', 'to_time', 'totime', 'to_interval', 'tointerval', 'regexp_match', 'now', '_now', 'age', 'year', 'month', 'week', 'day', 'hour', 'minute', 'second', 'day_of_week', 'title', 'levenshtein', 'longest_common_substring', 'hamming_distance', 'wordwrap', 'regexp_replace', 'regexp_substr', 'concat', 'strpos', '_left', '_right', 'rpad', 'lpad', 'format', 'format_number', 'format_date', 'color_rgb', 'color_rgba', 'ramp_color', 'color_hsl', 'color_hsla', 'color_hsv', 'color_hsva', 'color_cmyk', 'color_cmyka', 'color_part', 'darker', 'lighter', 'set_color_part', 'point_n', 'start_point', 'end_point', 'nodes_to_points', 'segments_to_lines', 'make_point', 'make_point_m', 'make_line', 'make_polygon', 'x_min', 'xmin', 'x_max', 'xmax', 'y_min', 'ymin', 'y_max', 'ymax', 'geom_from_wkt', 'geomFromWKT', 'geom_from_gml', 'relate', 'intersects_bbox', 'bbox', 'translate', 'buffer', 'point_on_surface', 'reverse', 'exterior_ring', 'interior_ring_n', 'geometry_n', 'bounds', 'num_points', 'num_interior_rings', 'num_rings', 'num_geometries', 'bounds_width', 'bounds_height', 'is_closed', 'convex_hull', 'sym_difference', 'combine', '_union', 'geom_to_wkt', 'geomToWKT', 'transform', 'uuid', '_uuid', 'layer_property', 'var', '_specialcol_', 'project_color']