For instance, the letter says somewhere around $3000, for one person and there are two letters for two separate people, but what was actually received for the EIP was only $1500 and that much was only received by one person? The numbers are completely off and I don't know what to do about it or how to fix it.
Check your online account with the IRS and see what it says there. If you filed a joint return they send you two letters and split the payments between the two of you. So you have to check for each spouse.
https://www.irs.gov/payments/your-online-account
The other thing might be that you are confusing the child tax credit payments with the EIP payments. If you have one child over the age of 5, you probably received $1500 between July and December. Those were not EIP payments. Those were advance child tax credit payments. You have letter 6419 for that.