Ok, so people say that SegWit is a soft fork. What does this mean?

So let's say I was running a version of Bitcoin Core that was downloaded and installed 2 years before SegWit activation. And then I just left it running without updating the software.

And then on SegWit activation day, blocks start coming in that contain Segwit transactions, eg. those bc1q addresses. So my 2-years outdated Bitcoin Core will accept those blocks as entirely valid and then add those blocks to my local Block Chain?

So what is to prevent me from doing my own soft fork? Let's call my soft fork SegCash, and then I buy a bunch of mining equipment and start mining my own blocks. Will those blocks that follow my own soft fork rules be accepted by all the other nodes?

I also heard people saying that SegWit is actually a block size increase. Wait what? So you can do a soft fork to increase the block size, and Bitcoin Core clients which are 2-years outdated will accept those blocks as valid and add those blocks to their Block Chain?

Am I understanding this correctly? Before SegWit, the Block Chain increases by 1 MB per 10 minutes on average. After SegWit, the Block Chain increases by 1 to 2MB per 10 minutes on average?

So what is to prevent me from doing my own soft fork and increase the block size to 100MB per block? And then I buy a bunch of mining equipment and mine my own blocks which follow my own soft fork rules, and then those blocks will be accepted by the rest of the network?

So if a miner were to start mining TapRoot blocks right now, those blocks will also be accepted by the rest of the network as valid?