Definitely beetle larvae but impossible to tell which kind from a photograph. A massive number in one location hints at May/June beetles, however. That's certainly how they behave in my location.

Save the pupa in some moist soil in a jar or baggie. They're easier to ID once they are adults.

They probably aren't good for your garden but if you never noticed anything wrong going on in the planter, they aren't doing anything bad, either.

Those are the larva of the very large Scarab beetle (30,000 members) family which includes May, June, and Japanese beetles. It is difficult to tell which larva belongs to which adult beetle even though there are descriptions of them on the web and most
ID involves counting hairs. They may be destructive now or as adults, but the time to control them is when the eggs hatch in August. Now is too late.

just lay them on the sidewalk or driveway ... the birds will make them disappear ...

ken