Cfrtjryk

From the sentence

He has been watching TV,

I understand that it is a present perfect continuous tense that started in the past and is continuing in the present time.
But I get confused at times when I hear sentences like

He has been involved.

Does it mean the same thing? That the involvement started in the past and is continuing in the present time?
If not, what is the difference?

You're right that the present perfect continuous started in the past and is continuing, but what makes that tense special is more of an emphasis on the duration or continuous course of the action. Both tenses are used to express that an action began in the past and is still going on or has just finished.

The present perfect simple tends to stand out as mainly being used to express that an action is completed or to emphasise the result.

But "to be" in this tense followed by an adjective or a state (not a verb gerund like "watching", but "happy" or "on holiday", for instance) is a special case and can only exist in this form because the key verb is "to be". It would be incorrect to say:

*He has been being...

This means that whether the action is continuing to the present moment or not, depends on the context or the implications that the verb allows. So, I can say:

I've been at work since 6AM.

And this implies that I am still at work right now.

But then again,

I've been worried sick about you!

doesn't imply that right this second I'm worried because I'm talking to the person and know they are safe. This tense is used because my worry started in the past, and continued up to the very recent present, but actually stopped just a moment ago.

For your example, "involving" yourself in something isn't the kind of thing you start and stop, but more of an ongoing state (depending heavily on the context though), so even if right this second he isn't doing something that means he is actively involved (like maybe he's eating or sleeping right now), he is still in general involved right now.

Both of your examples have the same general meaning but I believe your confusion may be a result of the context in which these statements are used.

He has been watching TV.

As you said, this is a statement about a continuing action that began sometime in the past as opposed to

He had been watching TV.

Which instead makes use of the past participle to indicate that the action has ended.

He has been involved in [a group, organization, or other subliminary action]

Suggests that this involvement may be ended or completed. There is ambiguity with the second example that can only be clarified through context.

Neither statement strongly implies that the activity continues in the present time, but merely suggests that, absent contextual clues, it might be continuing.

"He has been involved", in particular, is a common way to imply current involvement without claiming it -- "He has been involved in criminal activities" doesn't say he currently is involved, but leaves the listener to wonder. Often used as a sort of "smear".

And consider that "He has been watching TV" may be followed by "but now he's in the kitchen looking for a snack." There is no strong implication that the activity continues in the present time.

Compare to "had". When "has" is used the ending time of the activity is unspecified, and may continue at present or have ceased a few seconds prior, but when "had" is used the ending time is generally implied as a part of a timeline, and definitely ended some time in the past.