When my joining was delayed and there was no update in joining, I started looking into UPSC.
I got enrolled in VisionIAS, and used to attend online classes. The UPSC syllabus is very broad but the depth of things is not alot. You are expected to have a general understanding of everything. An in-depth understanding is good, but it's fine if you just know the basics.
The UPSC syllabus official notification also very clearly mentions this, that no specialized knowledge is expected for the Mains paper. Although your optional should be veryyyyyy strong.
The syllabus is broad, it includes History- Ancient, Mediaeval, and Modern. Focus is on Indian and some world history is also there. Art and Culture is also covered, you have Geography and both world and India. You have Science and Technology, International Relations, Biology - these are mostly current affairs based. My favourite is Polity though, fucking loved reading the constitution and devling into the depths of it.
I think a lot of this 'broad knowledge' just directly comes from reading the newspaper in depth daily. If you read the newspaper daily, especially the editorials, you'll be having this 'broad knowledge' too.
Now that I've started my job, I am sadly not able to dedicate a lot of to my prep. Let's see how it goes next...