Does a soft fork result in two different blockchain versions?
Your observation is correct, except that a fork is expected to be activated by a supermajority of nodes and miners in the network. The most recent soft fork had an activation threshold of 950 blocks in specific segments of 1,000 blocks signaling readiness (95% hashrate) for the new rules.
So, assuming that only 5% of the mining power don't follow the new rules:

Increasing block size: 95% of the mining power allows bigger blocks now, 5% continue to enforce the smaller limit. As soon as a block with the new rules is found, the old nodes that do not allow bigger blocks reject the main chain and start their own chain-tip. The minority chain moves much slower, and thus the majority of the miners and nodes ignores it. Assuming the split's proportions remain stable, the two chains remain irreconcilable. To mend the network split, one side would have to concede and adopt the other sides rules.

Decreasing block size: About 1 in 20 blocks will exceed the limit enforced by the majority. Only nodes still using the old rules will accept the bigger block, all other nodes ignore it. The majority of the hashrate will quickly outpace the big block minority. Since the small blocks are acceptable to the old nodes, big block nodes will reorganize to the small block chain as soon as that chaintip has more total proof of work. Since the difficulty between chaintips can only differ across difficulty readjustments, this generally happens whenever the small block chain pulls ahead by one block in height. As the big block now becomes part of a stale chaintip, the miner that authored the big block loses their block reward. In result, whenever a short chainfork with big blocks occurs, the network converges back on the smaller chain eventually. The big block miners are losing money unless they adopt the rules of the majority, or force the split to become permanent (essentially hardforking from the softforked majority).