Can I extend my 15-day Visa on Arrival in Thailand as an Indian passport holder?

As you’ve already been told, you can apply for a Covid extension and if you don’t get it, they will give you a 7 day denial stamp.

You have to ask Malaysian immigration if a denial stamp is considered a valid visa for them.

If not, check if they will accept a visa from Cambodia. If India gets a visa on arrival from Cambodia and Malaysia accepts that then you could spend 1-2 days there and use that to travel to Malaysia.

The first thing you need to know is that Visa on Arrival cannot be extended. The 7 days you mention is IF they let you apply for an extension (they may not let you), they will then deny the extension and stamp a 7 days to exit stamp in the passport. So it's not an extension, it's a denial stamp.

You MIGHT be able to apply for the 60 day Covid Extension. Once again they may or may not accept this application.

For Visa on Arrival in Bangkok, you would go to the office at IT Square LakSi Plaza. You are looking for Counter K on the 3rd floor.

Note that both Covid Extensions and 7 days to exit stamps are issued the same day of application, so if you go before your 15 days is over, you will lose any remaining days.