R. Yehuda says in the name of Rav that a T.C. is obligated to learn 3 things, writing, shechita and milah. Rashi explains that writing simply means that he must be able to sign his name for an eidus. Shechita means that he must know how to maneuver his hand to shecht properly and not just know the halachos (and the same for tying the knot of the tefillin). The Maharsha explains that a person has an obligation to learn and know the entire Torah, so Rashi is trying to emphasize the practical aspect of it - knowing the halachos of shechita isn't sufficient, rather one must be able to be מאמן ידו and do the shechita.
R. Chanina Bar Shalmei adds the knot of the tefillin, birchas chasanim and tzitzis. The gemara asks why the first opinion doesn't list those three and says - הני שכיחן. Rashi explains that since tefillin, birchas chasanim and tzitzis are common, everyone will automatically become an expert in them by osmosis without having to invest effort into learning how to do them. Rabbeinu Gershom offers a slightly different approach - since they are so common it is easy to find others who can do them. According to Rabbeinu Gershom, we have a justification nowadays for why we aren't obligated to become an expert in any of the 6 items. Since it is common to find people who are able to do these things (even the first 3), it is no longer necessary to make such an effort to be able to do them.
Tosafos seems to understand the gemara's comment of הני שכיחן like Rabbeinu Gershom. Tosafos equates this with what the gemara says that tefillin in the house of bar chavu are משכח שכיחי. Tosafos understands from here that since there are experts around who can do it, it is not essential that everyone else learn how to do it. The implication is that not everyone will know how to tie the knot of tefillin, but when needed one will be able to find someone to do it. From this Tosafos proves that the knot of the tefillin doesn't need to be retied every day as suggested by Rabbeinu Eliyahu, rather the requirement of וקשרתם just means that we must tighten it on our heads as Rabbeinu Tam explains. From the understanding of Rabbeinu Tam that the definition of וקשרתם is to tighten the strap around the head, it is imperative that it be tight and not loose since that is the definition of the mitzvah.
To respond to the proof of Tosafos, Rabbeinu Eliyahu may interpret the gemara of הני שכיחן like Rashi, which would mean that everyone will automatically become an expert since it is necessary to do every single day. According to Rabbeinu Eliyahu one wouldn't need to learn to be an expert in something that is routine and he would be doing every single day from the time he is 13.