> in Germany the employer owns all source code created during working hours by default.

Same in most countries I worked in. Generally it's not just the business hours, but also any kind of device used. If code got edited at one point on a company laptop, or during office hours, then it's the company's.

Most intellectual jobs in quant finance will also routinely enforce an intellectual property clause in their work contract, which extends appropriation to anything resulting from the knowledge you acquired at the company. Not sure if that is enforceable in practice, but it's always there.

What would be the general stance if I for example SSH'd into something I own and worked from there?

This AFAICT, isn't the case where I live, I usually have to sign something actively giving the the employer ownership (And usually it's scoped to the project), but I'm not sure if this is a formality