How did you learn to say no to work ?

Recently, I have been getting a lot of work that isn't even remotely related to my domain or skillset and in a complete opposite direction of what I do( because I don't have much workload on my main project)

I thought these idiots will appreciate what I am doing and will have boundaries or atleast guide me when I am stuck ( because this domain is completely different from my original skillset+ has no value in the market+ has very little documentation).

I have no interest in taking an extreme deep dive into this , but seems like they are pushing me to become an expert in this and gaslighting me into thinking I am dumb when I am stuck somewhere ( this is a very niche tool with not much documentation, I have tried 3 GPTs, all of them hallucinate stuff)

How do I set boundaries ( like I will work on thing if you need me to but won't deep dive into such an extent that it is a primary career )

( Basically I want to only work on the low code part , they are telling me to learn back-end part as well and upksill to such an extent I can deliver 10+ crore projects by myself ) . I was upskilling at a pretty good pace, I just got stuck in this backend part and have been at it for 20+ hours with no inroads, and these duckers are giving me no guidance