及其
jí qí · HSK 7
and
及 (reach/and) + 其 (that one) = and that as well; links a thing to its related items
及 means to reach or extend to, and 其 means that or its. Together 及其 connects a noun to whatever belongs with it, like saying and its as well in a list.
及 — hand 又 reaching person 人
Originally a pictograph showing a hand (又) catching or reaching a person from behind, depicting the act of catching up to someone. The core meaning 'to reach' or 'to catch up to' extended to 'and' (reaching/including more items) and 'in time for'. Modern form simplifies the person and hand into connected strokes.
其 — top 八 + winnowing basket 其 (originally 箕)
Originally a pictograph of a bamboo winnowing basket, now written 箕 with the bamboo radical added on top. The original form 其 was borrowed phonetically to write the pronoun and particle meaning 'his/her/its' or 'that.' This loan use became dominant, so a new character 箕 was created for the basket meaning.