I know that my ISP can detect when I am using a VPN, but can it see which protocol I am using?
Yes, probably. Most VPN protocols are not designed to hide the fact that they're VPN protocols, nor what kind of protocol they are. See for instance this paper which details fingerprinting OpenVPN.
If you run all your traffic through a VPN, the fact that you're using a VPN is quite easily visible - as all traffic will be destined for a single destination - which is quite unlike normal usage patterns.
If you're interested in hiding the fact that you're using a VPN, and what VPN, you should probably look into what's used in totalitarian states, such as China, which routinely blocks all attempts at VPNs out of China. Tor is one such system.
Yes they could, but they would need to do this actively. Some degree of effort is required. If you are one user in many that share an ISP connection there would need to be filtering, grouping going on. They also might not care. Your employer will care if you are breaking rules though. A disproportionate level of SSL traffic on 443 to a single address might be noticeable. (They have to keep some ports open).
But your ISP, unless they have a real reason to care, they won't. VPN's are pretty much mandatory for protecting personal information or joining your private network.
I daresay a very large number or people have been working from home lately. That's a lot of VPN traffic.
You probably have more to fear from a provider if it is not "your" VPN. They may say they don't "keep" logs, but that does not mean someone else is not archiving traffic.