Learn English – Which tense is being used in ‘it is moved from location A to B’

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.

Technically, the tense should match, so since we're using the perfect tense (conditional perfect in fact), you should use was.

But both could have ... is and could have ... was are acceptable in this case.

Why? Well, it's because your clause about the paprika being similar to the bell pepper might be true for a long time, and might continue to be true even in the present, so it might be okay to use is.

Maybe not. Maybe the paprika is rotten by now, or already eaten. Then is would make no sense. But a situation continuing to the present would justify use of the present tense is.

Consider:
"I could have gone shopping, because the supermarket was near."
"I could have gone shopping, because the supermarket is near."

If the supermarket has not suddenly moved, it probably still is near – it is now, and it was then – so either one is acceptable.

But some things do not last so long:
Correct: "I could have stayed longer, because it was early in the morning."
Incorrect: "I could have stayed longer, because it is early in the morning."

Unless you are describing something in the very recent past (minutes or hours ago), the fact that it is now early probably has nothing to do with the situation in the first half of that sentence, so mixing the past and present tense in this last example doesn't really work.