The belief is that moses wrote genesis through divine inspirations. Possibly based on earlier written records that existed before his time. Its nt so much abt moses witnessing the event of genesis, bt was guided to write it as larger narratives of israelite history n their relationship with God

He wrote it through divine intervention of God

Remember moses is a leader,he knows history too

It can be through history he wrote it with help of holy spirit

Holyspirit directed him to write