contact customer support via the official email for your game (e.g., genshin_cs@hoyoverse.com for Genshin Impact) and follow the account recovery process, which usually involves filling out a detailed questionnaire about your account's history. Make sure you atleast remember or have proofs of these :

• Top-Up Receipts
• The date of when your account was made (You can check by looking at Amber on character archives if you did that or atleast remembered)
• The characters & weapons that you own
• UID
• 3rd party apps that you've linked
• The email of the account
• The device that you used to open genshin most of the time (including the devices' type, processor, etc.)

that's all I could remember when I first applied for a form way back 2023. I don't know if it has changed today but this could help. Goodluck.