tense for past job experience?

You can, but simple past is more concise and more forceful.

For example: After I took this position, I obtained the Microsoft cert and the Cisco Networking cert.

People will assume that any certifications that you have obtained are still valid; if they were no longer valid, you would not mention them, right?

"after I started working for x co., i have had experiences on ... "

That is unidiomatic. Present perfect expresses what has happened up to the current time. When it's used to discuss a duration after some point in time, "since" is used, rather than "after". Also, in this context, "experience" should remain singular, and it takes the preposition "in", rather than "on".

In this context, "experience" refers to the state of having done a certain type of work. The work happened in the past, but the experience is present tense: you currently are in the state of having done the work. So you can say "I have experience in X from my position Y". You can also say "I gained experience in X from doing Y" or "I have gained experience in X since starting Y".

The present perfect means that you are choosing to present the events as having some present relevance: the exact meaning of that relevance is variable.

I have had experience on ... generally means that your experiences are over a period that includes the present.

A likely interpretation is that you are still in the relevant job:

Since I joined XYZ, I have had experience on ...

sets up a strong presumption that you are still there.

However, you could be using it over your career (a period that includes the present). If so you would need to choose words to make it clear that you were not referring to a single job, eg

From the time I was at XYZ, and subsequently, I have had experience on ...

But if you are talking about experience at a job that is in the past, the past simple is much more natural.