Let's say you wanted to build alternative structures to supply communities with life necessities that existing state and capital structures fail to provide.
Short of some revolutionary mass insurrection taking power, liberating capital resources, and upending the economic status quo; any attempts to build these alternative infrastructures would necessarily exist within, and interface with, currently existing capitalist systems. Let's start with the "modest" goal of providing food, dignified shelter to anyone who needs it; forever.
One assumption that is critical to this imagined pathway is that enough people with moderately high-incomes are not so far-gone that they would be willing to, if presented with the opportunity, pay more for things knowing they are directly subsidizing their neighbors.
Let's say you are able to somehow pull together 20 million dollars; maybe you have a lot of sympathetic patrons, maybe you really lucked out when the evil startup you worked for in your 20s IPO'd; maybe you inherited a bunch of money from someone who got it by being awful and you can't stand personally benefitting from it, maybe you just have a lot of friends who believe in you and share your vision.
Either way, you have some cash and the dream to try and boot up socialism from inside of a capitalistic operating system.
You use that money to start or buy a farm. Eventually, the goal will be to feed people directly with farming output, but for now you need to keep things self-sustaining, so the output of the farm gets sold. You pay the people working the farm fairly and avoid using the worst environmental practices. The farm turns a profit, but not a "maximal profit". You don't have shareholders, so you are allowed to do this.
These profits are split between donations to grassroots community organizations dedicated to feeding people, and reinvestment into the farming organization every year; continually growing outputs and increasing margins purely through scale while continuing to mind your environmental and labor responsibilities. You never pay yourself more than anyone else who is working for the project.
If this manages to become self-sustaining, you can take some of the excess and begin opening community cafeterias.
The goal of these cafeterias would be to efficiently serve healthy, desirable, meals each day from a daily menu that accounts for various dietary-restrictions in a friendly communal atmosphere.
Meals can be purchased a-la carte or through a subscription plan; The cost of a meal is on an income-based sliding scale; Diners with high-incomes pay more and directly subsidize lower-income diners. You remind the high-income diners of this as much as possible. Rich people love no-effort ways to feel like they are helping.
If someone is hungry, they get fed, no matter what.
Assuming the food supply side of the equation is able to grow sustainably; start buying mostly empty apartment buildings and converting them into housing co-ops. Since you own the building, you can work with some credit unions and extend low-interest mortgages to existing tenants at relatively low risk to yourself. They have the option to continue renting until they move out, but you structure the mortgages such that the monthly payments are lower than what the monthly rent to a landlord would be. There is also a monthly income-based co-op fee that would pay for maintenance and a blanket insurance policy for all units. This co-op fee would also go towards subsidizing several units from each property as housing-first opportunities for unhoused individuals and families. These opportunities would be provided without any pre-conditions such as seeking employment or entering into drug/mental health treatment.
Purchasing a unit comes with some strings, though; you must live in the unit as your primary residence for the majority of the year; if you move out, you are required to sell the unit, either back to the building at the fixed unit price, or to anyone else who wishes to buy it from you on the same terms. You can get a roommate, but you don't get to rent out the whole unit or collect properties as if they were pokemon cards. This is housing, not an investment.
Start replacing landlords with homeowners.
All the while, you are going to need to build ties. You will need to serve others wherever possible, and get people invested in the vision of this project. You will need to gain support and protection from people in power; Once you start to appear like a threat to capital, you will face attacks.