How sortition would work if there is a single account on the node?

What happens when I am running a node and there is only a single account with some Algos on that node. Does that mean only I will be selected as a proposer and a validator by Sortition?

Though to be finally elected as a proposer for the current block I should have the lowest credential compared to other proposer elected (through sortition by other nodes). Is that right or am I understanding it wrong?

A node can have multiple accounts.
But they all are completely independent.
Having multiple accounts on a single node is equivalent to running multiple nodes, one with each of those accounts.

Yes, at a high level, in normal conditions (in particular without network partitioning), to become a block proposer for a given block, you need to be selected as a potential block proposer and to have the lowest credential compared to other selected proposers.