Learn English – Correct verb tense for describing a movie

The best option would be:

I have joined a site which is focused on learning English.

Reason- You have joined the site and you are still in the site and you did not leave it. So the result of your last action (joining the site) still persists.

Why is Your No 1 option not Fully appropriate-

I joined a site does not imply that you are still in the site, which is not true. You are in it and you did not leave the site. You may still be in and it maybe that you have left the site lately which is not written.

Why Your No 2 Is Fully inappropriate-

which was focused on learning English. implies that the site was at one point, focused on learning English, but it is not focused on learning English now, which is wrong.

If you are learning English, I would suggest that you not follow any hard and fast rule for joining sentences. When you join sentences, just make sure the resultant sentence keeps the meaning of both the original sentences.

Consider a situation where you called on Tom at his home. The following day, you can say to a mutual acquaintance

Tom was at home yesterday

In this context, it is neutral about where Tom is now. He could be still at home: he could be somewhere else.

Now suppose that, when you were at Tom's house, you asked him where he came from, and he replied

I am from London.

When you tell your mutual acquaintance, you say to him

Tom said he was from London. - reported speech - backshifted
Tom is from London - direct statement

You can use is because Tom is definitely still from London.

Similarly, when you interview your client, you might get these answers:

My name is Tom
I am 34 years old
I work at XYZ bank
My background is in IT
I work in accounts

When you are writing up your report a few months later, you would be obliged to say

His name is Tom - still true
He was 34 years old - at the time of the interview
He worked at XYZ bank - at the time of the interview
His background is in IT - still true
He worked in accounts - at the time of the interview

I am not suggesting that you should always switch between present and past like this: it is just an example that shows how you can express these ideas precisely if required.

If it's not necessary to be precise and you have multiple items of information, it's probably best to stick to whichever is the most common of present or past: the context will usually be enough to avoid misunderstanding.

If precision is important, it depends exactly what you mean by "from the IT field". If it means "His training is in IT", then it's still true, so you should use is. If it means "He is currently working in IT", you should use was.