It's called a chroma key, you just chroma key the color green in the editing software of your choice and set the sensitivity low enough that the text remains but high enough that the text doesn't have jagged pixels of green.
what's your name?: Timです。 how can i call you?: ちむ! what kind of VTuber?: 人ー間ーでーす。 when's your birthday: 9/20 (きゅーにじゅ!) how old are you?: わかない what's your gender?: 男です!(だん) how tall are you?: わ。か。な。いー where are you from:? アメリカ! what's your charming point/good thing about you?: わかないです。 please do an impersonation: ハッハ!(✕ね)「ミッキー・マウス」 where are you active: Twitter! (ツイター) starting greeting: わかない ending greeting: わかない fan emojis?: わかない live tag?: わかない fan art tag?: わかない fan name?: わかない favorite color?: 青い what are your hobbies?: RU-vid!(ユーチューブ!↑) what are you good at?: 楽曲 (がっきょく) now confess your love!: 。。。ダメ。 favorite food?: Pizza & Hamburgers! favorite drink?: オレンジソーダ! favorite manga?: 僕とロボコ favorite anime?: Komi (古見)Can't Communicate! favorite game?: わかない favorite music?: わかない any dislikes?: 嫌 (stylized as: 嫌嫌嫌嫌(イヤイヤイヤイヤ)) who do you admire?: わかない what'(s) your dream?: わかない! some closing remarks,( )please♪: わかない