No. Jobs do not always increase welfare on average. You asked about new jobs but the distinction between new jobs and existing jobs is not very important from my point of view. Every job is new if you look at a vast expanse of history.
You can assume that if the restaurant market is over-saturated then a restaurant will go out of business. If
n restaurants exist, it means exactly
n restaurants will continue to operate in at least the short term and possibly the long term and that this is not the over-saturation level and that the market is over-saturated at
n+1 restaurants. If people are not forced to eat at a restaurant and the workers there are not forced to work then the transaction between the restaurant visitor and the restaurant worker increases the average welfare. You value the food at more than the price. The worker values the wages more than their labor. Both people benefit when there is no coercion involved.
If the number of restaurants increases
nnew ← nold + 1
and as always
nover-saturation = nexistent + 1
and restaurants are homogeneous (you are not replacing one large restaurant with 2 half-sized restaurants) then average welfare increased because customers increase welfare if no coercion was involved and workers increased welfare if no coercion was involved. This is true even if the economy did not grow and the population did not grow. It may be simply a change in preferences for eating away from home versus at home.
If coercion was involved in supporting the restaurant then it might not increase average welfare. If there is coercion perhaps people are forced or induced by the government to buy something at a restaurant so suddenly there are more restaurants and more restaurant workers. Since the people would rather have kept their money and avoided the restaurant meals in the absence of coercion, it is clear that they valued the money more than the meal so in the presence of coercion average welfare decreases. If a worker is forced to work in a restaurant when they are unwilling then there is a decrease in their welfare and since they are part of the average, the average welfare decreases.
For a more realistic example of coercion consider that people value the administration of justice (courts and judges). Taxes are collected by force to provide a justice system. The government controls the spending. If the government spends unwisely or corruptly then the value of taxes allocated to the administration of justice is greater than the value of the administration of justice. Government spending can be unwise because they do not have perfect information about the preferences of the taxpayers. It may be the taxpayers do not want the court house to be designed by an expensive world famous architect. Unfortunately you cannot purchase a small piece of the justice system for yourself at the price that reflects its value to you. Some purchases have to be coordinated by an imperfect government.
"I understand that you need to get people to spend their money
Perhaps we do not need to "get people to" do anything at all because they know their preferences best.
I would not assume that if a factory exports an object that it makes the exporting country richer. It is a misconception. If the labor and materials are worth 2 dollars to make the exported object but the export income generated by the object is 1 dollar (because there are many other countries selling the same thing for 1 dollar so you have no opportunity to raise the price) then your country has become poorer. Why would you export in this manner? Only when there is coercion involved will such an export happen. Perhaps the object was made in a factory that uses government subsidized electricity or a government subsidized interest rate was used to purchase the machinery or the workers are prison laborers. The business pays 0.75 dollars for materials but the subsidized inputs are worth 1.25 dollars for a total of 2 dollars. The object is exported for 1 dollar and since the business paid 0.75 dollars for its share of the inputs, the business makes a profit of 0.25 dollars. The taxpayers have a decrease in welfare. The unfortunate businesses that did not get a subsidized interest rate will complain about the part of their taxes that support the subsidy received by the other more fortunate businesses but there is little that they can do to change this. The subsidies are making the exporting country poorer and the importing country richer.
Is it the exporting country's subsidies that make the importing country richer? Not necessarily.
Even if there are no subsidies, the importing country is still getting richer. When there are no subsidies, the exporter stops producing and the importer buys the object from one of the other exporting countries at the market price of 1 dollar. The importing country might value the object at a value greater than 1 dollar so the importer experiences an increase in welfare. However, the market value is still 1 dollar so it appears that the importer's net worth did not increase. However, recognize that the importer's increase in welfare can be used to offset the need to spend money on an alternative means of generating the same amount of welfare. If we eliminate the need to make the alternative expenditure then the importer's net worth increased. Welfare and net worth are interchangeable when simplifying assumptions apply.
If the imported object is a consumable then the importer's net worth decreases when the consumption happens. Not all imports are consumable so importing isn't necessarily decreasing net worth. Consuming domestically produced consumables is also decreasing the net worth of the consumer so there is nothing inherent about the fact it was imported that decreases net worth.
If a thing (consumable or durable) was domestically produced there isn't necessarily more domestic output and more domestic jobs because if you import the thing then domestic workers are able to produce something else that is more rewarding. More importantly domestic workers are most certainly unable to produce something else more rewarding if the thing is induced by the government to be produced domestically.
To summarize: Exports do not necessarily increase wealth if the export is subsidized and imports do not necessarily reduce wealth; it is the subsequent consumption that reduces wealth if the object was consumable. Importing a thing can increase wealth if there are more rewarding opportunities than creating a thing that could be imported at sufficiently attractive price.