Welcome to KH!
You’ve just finished a knit row and you should have 3 sections of knitting on the needle with 2 gaps where 6sts were cast off. There should be 5sts, gap, 46sts, gap and 5sts. The working yarn is still attached to the end of the row. There’s no cast on yet.
Turn to the purl row, purl 5sts, drop the yarn, attach a new end of yarn to the middle 46sts and purl across, drop the yarn and attach another new end of yarn to the remaining 5sts, purl to the end.
You’ll work back and forth across these 3 sections, each with its own yarn source for 1.5". At the joining row you’ll cast on 6sts over the gaps where sts were originally bound off. This is a knit row. Knit 5, cast on 6, knit 46, cast on 6, knit 5. Now you’re back with one source of working yarn. The other 2 strands can be cut leaving a 6" tail to weave in later.
For the cast on in mid row you can use these instructions if you need help: