Is it necessary to start with the Dhuhr prayer when making up many missed prayers whose number is unknown, or is there no objection to starting with the Fajr prayer, and are the prayers made up previously valid?

Whoever has missed many prayers must make them up according to their ability until they are reasonably certain that their obligation has been fulfilled. They should make up the obligatory prayers, not the supererogatory or sunnah (definition) prayers, unless these are few. If they do not know the number of missed prayers, they must make them up until they are certain their obligation has been fulfilled. It is recommended for someone who has an unknown number of missed prayers to start by making up the Dhuhr prayer when performing the Qada', though it has also been said to start with the Fajr prayer, which is more preferable.