How is bitcoin (blockchain) secure? I see a few different levels here and I want to clarify
There's various levels where your password could be potentially exposed... at least from my mind

Storage (paper wallet, metal wallet, cold storage wallet)
Device (Phone, Computer etc.) i.e. Apple, Microsoft, Google (device makers)
Interface (Mobile App, Desktop App) (how do we know which are safe?)
Internet provider? (if it's https secure do we need to worry here?)
Miner? (is it at all possible for miners to manipulate your transaction?)

On what levels are we the most vulnerable?
The question is a little confusing, but:
1 and 2 are more to do with your own personal security practices. Of course, your own bitcoins are only as safe as the measures you take to secure them.
#3 - It is recommended to stick to wallet software that is open source, such that its code can be audited and verified to be safe. If you can read code, you are encouraged to read through the code of the wallet software you use and compile it yourself.
#4 - Traffic on the Bitcoin network isn't encrypted at all. If you run a Bitcoin node, your ISP will be able to see that you're running a Bitcoin node, and if you relay transactions from your node, your ISP will be able to assume those are your transactions. However, they won't be able to modify them, as any modification would require a new signature with your private key.
#5 - At worst, a miner launching a 51% attack can prevent your transactions from confirming. While harmful, they won't be able to actually manipulate your transaction, as your transaction still must have a valid signature from your private key.