It can be hard sometimes to know whether a noun is proper or common; sometimes even the same word can have both types! (A "house" is a common noun, a place to live; but the "House" could be short for a section of the US (or UK as well?) government, and "House" by itself is a TV show.)
The general idea is simple: Proper nouns are when there's just one of this thing. Common nouns are for when we're talking about a type of thing; we might be talking about one of this kind of thing, but there are others of this type.
So the first step is to think about the thing that you're naming. Look it up if you need to. Is it something that there's just one of, or that there are many of? For instance, you never capitalized "website"; that's an easy common noun. Think about "car loans"; there are many car loans, and many car loan companies. And just because "telecomm" and "fintech" combine words doesn't make them proper nouns. This can be confirmed by looking them up in a dictionary.
Brand names and trademarks are proper nouns. There's just one Amazon company (and just one Amazon river!). Looking up the official website for Git, I find they capitalize it in phrases like "the advantages of Git." (Though it gets confusing since typing git commands on the computer usually uses lowercase, so the logo at the top of the page does as well. I confess I usually use lowercase when talking about it casually at work.) I'm not sure we can call Git a "brand" when it's open-source, but it is a trademark. "JavaScript" is also trademark (held by Oracle), but you've noticed it has its own "camel case" style of capitalization. When we're writing generally, we can decide how faithful we want to be to proper nouns that want unusual capitalization, like e e cummings or will.i.am, eBay, and IKEA, but if we ignore their preferences we should at least capitalize the first letter.
Now, this can get confusing because trademarks and brand names can also be used to refer to products made under that name, and there can be many copies of a product. The easiest example is car models. When I have a model of car by Honda called a Civic, I don't have the only one. But I still capitalize it, "I have a Civic." People could argue a long time over whether this is a proper or common noun; I would say both. There is a proper noun sense of "the Civic," when we're talking about the entire concept and phenomenon, as in "Honda promoted the Civic heavily." I would say that we capitalize because we're referencing that idea, and I would say that we're using "Civic" to stand for a much more common word, car. So we're using a proper noun to do the work of a common noun. There's something similar in "I bought a Monet"—there's just one Monet, the painter, but we really mean "I bought a painting by Monet." Similarly, I see arguments that demonyms (names for people from a place, like "Nevadans" from Nevada) are proper nouns. I'm unconvinced by this argument, and would argue similarly that they're adjectives derived from proper nouns doing the work of common nouns... But arguments aside, we capitalize them because of the proper noun they refer to.
Note that we also usually capitalize abbreviations. So UI, HTML, etc. are not proper nouns, but are rightly capitalized. (But note, the style tool "Sass" is often mistaken as "SASS," but it's not, it's a single word!)
development of websites... JS instead of JavaSript ... the first sentence should be two sentences ... the last sentence should also be two sentences