Hello, Hoping for some clarity. If a baby is under 7 months old, and can roll, is it safe to be sleeping in a sleep bag (arms not covered) or should they only be in a onesie? Thank you

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

If your baby can roll well (ie both ways) is in a cot (no longer in a bassinet) & has arms free - continue to place on back for sleep & allow to stay in their position of choice.
A sleeping bag (arms out) is fine for a baby when rolling - often now babies will not stay under any bedding.
Ensure mattress is firm, fits well & is flat (not elevated) & no loose or soft, padded bedding